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).
> consistent with the other flags

Do you have any in mind?

As far as I know, cross-flag relationships *are* documented, e.g. `--only_closure_dependencies`: "this supersedes `manage_closure_dependencies`"

`--compilation_level=SIMPLE` and `--compilation_level=SIMPLE --process_closure_primitives` are actually the same behavior, but AFAIK currently not clear anywhere that this is the case.