Add exception hierarchy to the documentation.

docs/ext/commands/api.rst

@@ -214,8 +214,8 @@ Converters
 
 .. _ext_commands_api_errors:
 
-Errors
--------
+Exceptions
+-----------
 
 .. autoexception:: discord.ext.commands.CommandError
     :members:
@@ -265,3 +265,25 @@ Errors
 .. autoexception:: discord.ext.commands.BotMissingPermissions
     :members:
 
+Exception Hierarchy
++++++++++++++++++++++
+
+.. exception_hierarchy::
+
+    - :exc:`~.DiscordException`
+        - :exc:`~.commands.CommandError`
+            - :exc:`~.commands.ConversionError`
+            - :exc:`~.commands.UserInputError`
+                - :exc:`~.commands.MissingRequiredArgument`
+                - :exc:`~.commands.TooManyArguments`
+                - :exc:`~.commands.BadArgument`
+                - :exc:`~.commands.BadUnionArgument`
+            - :exc:`~.commands.CommandNotFound`
+            - :exc:`~.commands.CheckFailure`
+                - :exc:`~.commands.NoPrivateMessage`
+                - :exc:`~.commands.NotOwner`
+                - :exc:`~.commands.MissingPermissions`
+                - :exc:`~.commands.BotMissingPermissions`
+            - :exc:`~.commands.DisabledCommand`
+            - :exc:`~.commands.CommandInvokeError`
+            - :exc:`~.commands.CommandOnCooldown`