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_client.rst

@@ -254,7 +254,7 @@ Attempts to delete a message
 - **message** - The Message_ to delete
 - **options** - `object` containing the following:
     - **wait** - Milliseconds as a `number` to wait before deleting the message
-- **callback**
+- **callback** - `function` that takes the following parameters:
     - **error** - error object if any occurred
 
 getChannelLogs(channel, `limit`, `options`, `callback`)
@@ -430,7 +430,7 @@ deleteInvite(invite, `callback`)
 
 Deletes an invite
 
-- **invite** - An `Invite ID Resolvable`_
+- **invite** - An `Invite Resolvable`_
 - **callback** - a `function` taking the following:
     - **error** - error if any occurred