[docs][llvm-nm] Improve symbol code documentation

The existing symbol code documentation was very incomplete. This patch
adds the missing codes, and defines them based on the current code
behaviour.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42231.

Reviewed by: rupprecht, mtrent, MaskRay

Differential Revision: https://reviews.llvm.org/D63327

llvm-svn: 364171