Some doc fixes (#315)

#docs_client.rst:
- 257: Add proper eplanation of callback, like in every other method
- 433: Fix linking for Invite Resolvable

#docs_message.rst:
- 90 : Replace ':' with '-', just for consistency with the others

#docs_resolvables.rst:
- 92 : String doesn't link to anything (?) -> Link removed

#docs_servers.rst:
- 109 : Properly indent code-block because it didn't show at all on rtfd

#docs_serverchannel.rst:
- 53 : Remove doulbe '`' causing it not to display properly

#docs_voiceconnection.rst:
- 67 : Remove 'currently, it currently

#docs_voiceconnection.rst:
- 32 : Add links for Role Resolvable, as used in client#addMemberToRole,
client#removeMemberFromRole and client#memberHasRole

docs/docs_message.rst

@@ -87,4 +87,4 @@ isMentioned(user)
 
 Returns true if the given user was mentioned in the message.
 
-- **user** : A `User Resolvable`_
+- **user** - A `User Resolvable`_