[docs][llvm-readobj] Add a note to options that do nothing in GNU output

[docs][llvm-readobj] Add a note to options that do nothing in GNU output

--section-data, --section-relocations and --section-symbols have no
effect for GNU style ouput. This patch changes the docs to point this
out, as it has caught me out on a couple of occasions.

See also https://bugs.llvm.org/show_bug.cgi?id=42522.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365221 91177308-0d34-0410-b5e6-96231b3b80d8