ada: Fix documentation of -gnatwc

-gnatwc has been correctly emitting warnings for expressions outside
of tests for a while, but its documentation in the user's guide had
never been updated to reflect that. Also, the documentation used
"conditional expressions" to designate boolean expressions, but
"conditional expressions" has been defined by Ada 2012 to designate
if expressions and case expressions. This patch fixes those issues.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst: Fix
	-gnatwc documentation.
	* gnat_ugn.texi: Regenerate.