Correct incorrect text and clarify unclear text that has been identified
in the "Validity Checking" section of the GNAT UG.

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

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Improve -gnatVa, -gnatVc, -gnatVd, -gnatVe, -gnatVf, -gnatVo,
	-gnatVp, -gnatVr, and -gnatVs switch descriptions.
	* gnat_ugn.texi: Regenerate.