Update Usage messages to conform to new Usage message contributor guidelines (#3180)

CONTRIBUTING.md

@@ -215,3 +215,28 @@ Keep the element on the stack if possible. If you need a pointer or have complex
 - Use the [object tree](https://doc.qt.io/qt-5/objecttrees.html#) to manage lifetimes where possible. Objects are destroyed when their parent object is destroyed.
 - If you have to explicitly delete an object use `variable->deleteLater()` instead of `delete variable`. This ensures that it will be deleted on the correct thread.
 - If an object doesn't have a parent, consider using `std::unique_ptr<Type, DeleteLater>` with `DeleteLater` from "src/common/Common.hpp". This will call `deleteLater()` on the pointer once it goes out of scope, or the object is destroyed.
+
+## Conventions
+
+#### Usage strings
+
+When informing the user about how a command is supposed to be used, we aim to follow [this standard](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html) where possible.
+
+- Square brackets are reserved for `[optional arguments]`.
+- Angle brackets are reserved for `<required arguments>`.
+- The word _Usage_ should be capitalized and must be followed by a colon.
+- If the usage deserves a description, put a dot after all parameters and explain it briefly.
+
+##### Good
+
+- `Usage: /block <user>`
+- `Usage: /unblock <user>. Unblocks a user.`
+- `Usage: /streamlink <channel>`
+- `Usage: /usercard <user> [channel]`
+
+##### Bad
+
+- `Usage /streamlink <channel>` - Missing colon after _Usage_.
+- `usage: /streamlink <channel>` - _Usage_ must be capitalized.
+- `Usage: /streamlink channel` - The required argument `channel` must be wrapped in angle brackets.
+- `Usage: /streamlink <channel>.` - Don't put a dot after usage if it's not followed by a description.

src/controllers/commands/CommandController.cpp

@@ -296,7 +296,7 @@ void CommandController::initialize(Settings &, Paths &paths)
     auto blockLambda = [](const auto &words, auto channel) {
         if (words.size() < 2)
         {
-            channel->addMessage(makeSystemMessage("Usage: /block [user]"));
+            channel->addMessage(makeSystemMessage("Usage: /block <user>"));
             return "";
         }
 
@@ -341,7 +341,7 @@ void CommandController::initialize(Settings &, Paths &paths)
     auto unblockLambda = [](const auto &words, auto channel) {
         if (words.size() < 2)
         {
-            channel->addMessage(makeSystemMessage("Usage: /unblock [user]"));
+            channel->addMessage(makeSystemMessage("Usage: /unblock <user>"));
             return "";
         }
 
@@ -455,7 +455,7 @@ void CommandController::initialize(Settings &, Paths &paths)
         if (words.size() < 2)
         {
             channel->addMessage(
-                makeSystemMessage("Usage /user [user] (channel)"));
+                makeSystemMessage("Usage: /user <user> [channel]"));
             return "";
         }
         QString userName = words[1];
@@ -625,7 +625,7 @@ void CommandController::initialize(Settings &, Paths &paths)
                 (!channel->isTwitchChannel() || channel->isEmpty()))
             {
                 channel->addMessage(makeSystemMessage(
-                    "Usage: /streamlink [channel]. You can also use the "
+                    "Usage: /streamlink <channel>. You can also use the "
                     "command without arguments in any Twitch channel to open "
                     "it in streamlink."));
                 return "";
@@ -646,7 +646,7 @@ void CommandController::initialize(Settings &, Paths &paths)
                 (!channel->isTwitchChannel() || channel->isEmpty()))
             {
                 channel->addMessage(makeSystemMessage(
-                    "Usage: /popout [channel]. You can also use the command "
+                    "Usage: /popout <channel>. You can also use the command "
                     "without arguments in any Twitch channel to open its "
                     "popout chat."));
                 return "";
@@ -673,7 +673,7 @@ void CommandController::initialize(Settings &, Paths &paths)
         if (words.size() < 2)
         {
             channel->addMessage(
-                makeSystemMessage("Usage: /settitle <stream title>."));
+                makeSystemMessage("Usage: /settitle <stream title>"));
             return "";
         }
         if (auto twitchChannel = dynamic_cast<TwitchChannel *>(channel.get()))
@@ -704,7 +704,7 @@ void CommandController::initialize(Settings &, Paths &paths)
         if (words.size() < 2)
         {
             channel->addMessage(
-                makeSystemMessage("Usage: /setgame <stream game>."));
+                makeSystemMessage("Usage: /setgame <stream game>"));
             return "";
         }
         if (auto twitchChannel = dynamic_cast<TwitchChannel *>(channel.get()))
@@ -769,7 +769,7 @@ void CommandController::initialize(Settings &, Paths &paths)
                                          const ChannelPtr channel) {
         if (words.size() < 2)
         {
-            channel->addMessage(makeSystemMessage("Usage: /openurl <URL>."));
+            channel->addMessage(makeSystemMessage("Usage: /openurl <URL>"));
             return "";
         }
 

src/providers/twitch/IrcMessageHandler.cpp

@@ -810,8 +810,7 @@ void IrcMessageHandler::handleNoticeMessage(Communi::IrcNoticeMessage *message)
         if (tags == "bad_delete_message_error" || tags == "usage_delete")
         {
             channel->addMessage(makeSystemMessage(
-                "Usage: \"/delete <msg-id>\" - can't take more "
-                "than one argument"));
+                "Usage: /delete <msg-id>. Can't take more than one argument"));
         }
         else if (tags == "host_on" || tags == "host_target_went_offline")
         {