[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

docs/CommandGuide/llvm-readelf.rst

@@ -163,7 +163,8 @@ OPTIONS
 .. 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
 

docs/CommandGuide/llvm-readobj.rst

@@ -214,6 +214,12 @@ The following options are implemented only for the ELF file format.
 
  Display the section to segment mapping.
 
+.. 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. Currently only implemented for GNU
+ style output.
+
 .. option:: --version-info, -V
 
  Display version sections.