Add --process_closure_primitives behavior to --compilation_level description

Also fix a couple typos.

It is nice to know that `--compilation_level ADVANCED --process_closure_primitives` is redundant. Plus it simplifies other documentation like in the [Closure Library](https://github.com/google/closure-library/pull/312).
@concavelenz, true by default only for `SIMPLE` and `ADVANCED`, right?