Fix missing or broken versionadded in docstrings

This also documents BadFlagArgument.flag

discord/channel.py

@@ -785,6 +785,8 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
 
         This includes both private and public threads.
 
+        .. versionadded:: 2.0
+
         Raises
         ------
         HTTPException

discord/client.py

@@ -328,6 +328,8 @@ class Client:
         If this is not passed via ``__init__`` then this is retrieved
         through the gateway when an event contains the data. Usually
         after :func:`~discord.on_connect` is called.
+        
+        .. versionadded:: 2.0
         """
         return self._connection.application_id
 
@@ -335,7 +337,7 @@ class Client:
     def application_flags(self) -> ApplicationFlags:
         """:class:`~discord.ApplicationFlags`: The client's application flags.
 
-        .. versionadded: 2.0
+        .. versionadded:: 2.0
         """
         return self._connection.application_flags  # type: ignore
 
@@ -734,6 +736,8 @@ class Client:
 
         This is useful if you have a channel_id but don't want to do an API call
         to send messages to it.
+        
+        .. versionadded:: 2.0
 
         Parameters
         -----------
@@ -744,7 +748,7 @@ class Client:
 
         Returns
         --------
-        :class:`PartialMessageable`
+        :class:`.PartialMessageable`
             The partial messageable
         """
         return PartialMessageable(state=self._connection, id=id, type=type)
@@ -1575,6 +1579,8 @@ class Client:
 
         This method should be used for when a view is comprised of components
         that last longer than the lifecycle of the program.
+        
+        .. versionadded:: 2.0
 
         Parameters
         ------------
@@ -1604,5 +1610,8 @@ class Client:
 
     @property
     def persistent_views(self) -> Sequence[View]:
-        """Sequence[:class:`.View`]: A sequence of persistent views added to the client."""
+        """Sequence[:class:`.View`]: A sequence of persistent views added to the client.
+        
+        .. versionadded:: 2.0
+        """
         return self._connection.persistent_views

discord/ext/commands/errors.py

@@ -931,6 +931,14 @@ class TooManyFlags(FlagError):
 class BadFlagArgument(FlagError):
     """An exception raised when a flag failed to convert a value.
 
+    This inherits from :exc:`FlagError`
+
+    .. versionadded:: 2.0
+
+    Attributes
+    -----------
+    flag: :class:`~discord.ext.commands.Flag`
+        The flag that failed to convert.
     """
     def __init__(self, flag):
         self.flag = flag

discord/message.py

@@ -1485,6 +1485,8 @@ class Message(Hashable):
         :attr:`~discord.Permissions.use_threads` in order to create a thread.
 
         The channel this message belongs in must be a :class:`TextChannel`.
+        
+        .. versionadded:: 2.0
 
         Parameters
         -----------

docs/api.rst

@@ -739,6 +739,8 @@ to handle it, which defaults to print a traceback and ignoring the exception.
 
     This requires :attr:`Intents.members` to be enabled.
 
+    .. versionadded:: 2.0
+
     :param member: The member who joined or left.
     :type member: :class:`ThreadMember`