ada: Extend and clarify documentation of stack size settings for Windows

ada: Extend and clarify documentation of stack size settings for Windows

The original documentation for more recent versions of Windows didn't specify
whether the specified stack size acts as a "reserved" or "committed" stack
size.

Also, clarify the wording for older versions of Windows.

gcc/ada/ChangeLog:

	* doc/gnat_ugn/platform_specific_information.rst
	(Setting Stack Size from gnatlink): Improve documentation.
	* gnat-style.texi: Regenerate.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.