Custom OAuth - documentation/help text improvements

[torgeirl]

Description:

The Custom OAuth menu is currently without any help text, and rocket.chat docs doesn't mention how to set one up so part from the Callback URL the menu provide you're pretty much left figuring it out yourself.

Suggested improvements:

• Add help text to fields that needs more explanation. Ie. it isn't very intuitive that the Scope field takes a space-separated list of scopes (maybe rename to 'Scope(s)' would be appropriate), or how the Username field is mapped (from what I can tell you can tell where to look for values named 'username', but not a field called something else)
• Add documentation on Custom OAuth to the Rocket.Chat Docs

[Dhiraj240]

@MartinSchoeler @MarcosSpessatto
Can you assign me?

[marceloschmidt]

We welcome every pull-request with help in writing our documentation.