Correct some missing cases of setting this switch, document it in
usage. Primarily documentation, so no new test needed. Also makes
clear that this switch is not available in VMS. No test needed.

Tested on x86_64-pc-linux-gnu, committed on trunk

2013-01-04  Robert Dewar  <dewar@adacore.com>

	* gnat_ugn.texi: Document -gnatw.d/w.D (does no apply in VMS mode).
	* usage.adb: Add lines for -gnatw.d/w.D switches.
	* warnsw.adb: Minor fixes (some missing cases of setting
	Warning_Doc_Switch). Reject -gnatw.d and -gnatw.D in VMS mode.