Clarify the documentation for Stack_Usage (both comments in the code,
and user documentation) to note that tools like Valgrind won't work.

2019-09-17  Bob Duff  <duff@adacore.com>

gcc/ada/

	* doc/gnat_ugn/gnat_and_program_execution.rst: Clarify
	documentation.
	* gnat_ugn.texi: Regenerate.
	* libgnat/s-stausa.ads: Clarify comments.

From-SVN: r275787