[docs][llvm-nm] Add missing options to documentation

There were several options missing from the documentation. This patch
adds them as well as improving some wording and separating the Mach-O
only options into a separate section.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42234.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D63655

llvm-svn: 364176