ada: Document supported GCC optimization switches

In particular the most recently added ones, namely -Og and -Oz.  But -Ofast
is not documented because it disregards strict compliance with standards.

gcc/ada/ChangeLog:

	* usage.adb (Usage): Justify the documentation of common switches
	like that of other switches.  Rework that of the -O switch.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst (Compiler
	switches) <-O>: Rework and document 'z' and 'g' operands.
	* doc/gnat_ugn/gnat_and_program_execution.rst (Optimization Levels):
	Rework and document -Oz and -Og switches.
	* gnat_ugn.texi: Regenerate.