Docs for the Completer API
Actually stopping compilation when no support

Previously we would just emit an error message _and continue_. The "and
continue" part was an embarrassing oversight.
The docs now reflect the new omni-completer

... and the new, fleshed-out Completer API.

YCM now provides semantic completions for every language that Vim has
omnicomplete support for, which is _a lot of them_.