Change yield from to await in some docstrings.

Fix #625.

discord/client.py

@@ -3027,7 +3027,7 @@ class Client:
             overwrite = discord.PermissionOverwrite()
             overwrite.read_messages = True
             overwrite.ban_members = False
-            yield from client.edit_channel_permissions(message.channel, message.author, overwrite)
+            await client.edit_channel_permissions(message.channel, message.author, overwrite)
 
         Parameters
         -----------

discord/voice_client.py

@@ -367,7 +367,7 @@ class VoiceClient:
 
         Basic usage: ::
 
-            voice = yield from client.join_voice_channel(channel)
+            voice = await client.join_voice_channel(channel)
             player = voice.create_ffmpeg_player('cool.mp3')
             player.start()