[Docs] More warning fixes to unbreak the docs buildbot.

A number of warnings still remain, but these were the last of the
"unlexable code"-related ones (AFAICT).

I changed a few examples in docs/UsersManual.rst to showcase
-Wextra-tokens because it's already documented (-Wmultichar isn't), and
the sphinx C lexer apparently can't handle char literals like 'ab'. It
seemed like a better overall approach than just marking the code blocks
as none or console.

llvm-svn: 273232