[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.

llvm-svn: 365221
diff --git a/llvm/docs/CommandGuide/llvm-readobj.rst b/llvm/docs/CommandGuide/llvm-readobj.rst
index bb08279..4999b22 100644
--- a/llvm/docs/CommandGuide/llvm-readobj.rst
+++ b/llvm/docs/CommandGuide/llvm-readobj.rst
@@ -101,15 +101,18 @@
 
 .. option:: --section-data, --sd
 
- When used with :option:`--sections`, display section data for each section shown.
+ When used with :option:`--sections`, display section data for each section
+ shown. This option has no effect for GNU style output.
 
 .. option:: --section-relocations, --sr
 
- When used with :option:`--sections`, display relocations for each section shown.
+ When used with :option:`--sections`, display relocations for each section
+ shown. This option has no effect for GNU style output.
 
 .. option:: --section-symbols, --st
 
  When used with :option:`--sections`, display symbols for each section shown.
+ This option has no effect for GNU style output.
 
 .. option:: --stackmap