[lld-macho][nfc] Fix test to reflect that symbol attributes don't matter within an archive

We had a comment that claimed that defined symbols had priority
over common symbols if they occurred in the same archive. In fact, they
appear to have equal precedence. Our implementation already does this,
so I'm just updating the test comment. Also added a few other test
comments along the way for readability.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D106595