[docs][llvm-readelf][llvm-readobj] Improve --stack-sizes documentation

[docs][llvm-readelf][llvm-readobj] Improve --stack-sizes documentation

llvm-readobj's document was missing --stack-sizes entirely from its
document, so this patch adds it. It also adds a note to the llvm-readelf
description that the switch is only implemented for GNU style output
currently. For reference, --stack-sizes was added in r367942.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D67548

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371862 91177308-0d34-0410-b5e6-96231b3b80d8