ada: Improve documentation for -gnatw.h warnings

ada: Improve documentation for -gnatw.h warnings

The -gnatw.h option enables warnings about "gaps" in record layout
specifications. In the case of a "partial" layout specification, where the
locations of some components are left unspecified, the resulting warnings
may be incomplete or incorrect. Document this implementation limitation.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst: Improve
	the description of how the -gnatw.h switch interacts with
	"partial" record layout specifications (i.e., specifications where
	the locations of some components are left unspecified).
	* gnat_ugn.texi: Regenerate.