[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

llvm-svn: 371862
diff --git a/llvm/docs/CommandGuide/llvm-readelf.rst b/llvm/docs/CommandGuide/llvm-readelf.rst
index ecdb8f8..6aa9047 100644
--- a/llvm/docs/CommandGuide/llvm-readelf.rst
+++ b/llvm/docs/CommandGuide/llvm-readelf.rst
@@ -163,7 +163,8 @@
 .. option:: --stack-sizes
 
  Display the contents of the stack sizes section(s), i.e. pairs of function
- names and the size of their stack frames.
+ names and the size of their stack frames. Currently only implemented for GNU
+ style output.
 
 .. option:: --string-dump=<section[,section,...]>, -p