[Ada] Make it clear that gnatmake passes the ball to gprbuild if -P is set

[Ada] Make it clear that gnatmake passes the ball to gprbuild if -P is set

Also move -P switch description to the top of the switches list.

gcc/ada/

	* makeusg.adb,
	doc/gnat_ugn/building_executable_programs_with_gnat.rst: Move -P
	to the top of switches list and make it clear that gnatmake
	passes the ball to gprbuild if -P is set.
	* gnat_ugn.texi: Regenerate.