diff --git a/discord/ext/commands/bot.py b/discord/ext/commands/bot.py
index ee38bfc8b..7157319d5 100644
--- a/discord/ext/commands/bot.py
+++ b/discord/ext/commands/bot.py
@@ -39,13 +39,17 @@ from .errors import CommandNotFound, CommandError
 from .formatter import HelpFormatter
 
 def when_mentioned(bot, msg):
-    """A callable that implements a command prefix equivalent
-    to being mentioned, e.g. ``@bot ``."""
+    """A callable that implements a command prefix equivalent to being mentioned.
+
+    These are meant to be passed into the :attr:`.Bot.command_prefix` attribute.
+    """
     return [bot.user.mention + ' ', '<@!%s> ' % bot.user.id]
 
 def when_mentioned_or(*prefixes):
     """A callable that implements when mentioned or other prefixes provided.
 
+    These are meant to be passed into the :attr:`.Bot.command_prefix` attribute.
+
     Example
     --------
 
@@ -53,6 +57,19 @@ def when_mentioned_or(*prefixes):
 
         bot = commands.Bot(command_prefix=commands.when_mentioned_or('!'))
 
+
+    .. note::
+
+        This callable returns another callable, so if this is done inside a custom
+        callable, you must call the returned callable, for example:
+
+        .. code-block:: python3
+
+            async def get_prefix(bot, message):
+                extras = await prefixes_for(message.guild) # returns a list
+                return commands.when_mentioned_or(*extras)(bot, message)
+
+
     See Also
     ----------
     :func:`.when_mentioned`