Improve YCM contrib.

YCM was signaling an erroneous warning `no newline at end of file` that was bothering me much.
I discussed it with @Valloric some time ago (https://github.com/Valloric/YouCompleteMe/issues/950), and the culprit turned out to be libclang, not YCM. @Valloric gave me the keys to understand the problem and fixing it. This solves the issue by passing additional flag -Wno_newline_eof to YCM.

Documentation is also slightly improved.