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).
Most of the flags are meaningless in whitespace only mode (which simply parses and prints) as none of the compiler passes have a chance to run. So simply "true by default" is sufficient to describe the behavior.  This is consistent with the other flags.