[llvm-readobj][test] - unwind.test: add comments, document the current behavior.

Here I've added comments, added testing for llvm-readelf and documented
the behavior that we already have.

It was discussed in the D80380 thread that we want to improve the
"p_memsz does not match p_filesz for GNU_EH_FRAME" message reported
(and probably convert error to a warning). This patch is a preparation
for that.

Differential revision: https://reviews.llvm.org/D80635