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).
It would be more accurate to say "WHITESPACE_ONLY" disables all warnings and code transformations.  We don't want to have to tag every option as "requires SIMPLE or ADVANCED mode".