Add YARD docs to Faker::String

[danielTiringer]

Issue#

#1762

Description:

Add complete YARD docs for Faker::String methods.

[danielTiringer]

Some of the symbols listed in the documentation's examples aren't ascii.
Rubocop doesn't like that: Style/AsciiComments: Use only ascii symbols in comments.
Please advise if you think I should prune those out or even replace with something complex-looking but compliant. Or turn off rubocop for those lines.

[Zeragamba]

I think it's important to show that utf8 characters are returned, and I would prefer to see their visual representation instead of an encoded one. I think we can ask robocop to back off a bit there.

[Zeragamba]

Looks good to me. We should have a looks to see if we can tell YARD to ignore robocop commands.