[Ada] Adjust documentation of -gnatn switch

[Ada] Adjust documentation of -gnatn switch

This changes the wording in the documentation of the -gnatn switch to make
it use "units" rather than "modules" and also adjusts the usage message.

No functional changes.

2018-05-29  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst (Alphabetical
	List of All Switches): Replace "modules" with "units".
	(Subprogram Inlining Control): Likewise.
	* gnat_ugn.texi: Regenerate.
	* usage.adb (Usage): Fix description of -gnatn switch.

From-SVN: r260889