ada: Atomic_Synchronization is not a user-visible check

Remove all user-level documentation of the check name
"Atomic_Synchronization". The documentation was confusing because
this check should never be used in source code, and because it
raises the question of whether All_Checks applies to it (it does
not).

Change the name Atomic_Synchronization to be _Atomic_Synchronization
(with a leading underscore) so that it cannot be used in source code.

This "check" is not really a check at all; it is used only internally in
the implementation of Disable/Enable_Atomic_Synchronization, because the
placement and scope of these pragmas match pragma Suppress.

gcc/ada/ChangeLog:

	* doc/gnat_rm/implementation_defined_characteristics.rst:
	Remove Atomic_Synchronization.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Likewise.
	* doc/gnat_rm/implementation_defined_pragmas.rst: DRY.
	Consolidate documentation of Disable/Enable_Atomic_Synchronization.
	* checks.adb: Comment fix.
	* exp_util.ads: Likewise.
	* targparm.ads: Likewise.
	* types.ads: Likewise.
	* gnat1drv.adb: Likewise. DRY.
	* sem_prag.adb (Process_Disable_Enable_Atomic_Sync):
	Change name of Atomic_Synchronization to start with
	underscore.
	(Process_Suppress_Unsuppress): No need to check Comes_From_Source for
	Atomic_Synchronization anymore; _Atomic_Synchronization can never
	come from source. (Anyway, it shouldn't be ignored; it should be
	an error.)
	* snames.ads-tmpl (Atomic_Synchronization):
	Change name to start with underscore.
	* switch-c.adb (Scan_Front_End_Switches):
	Minor cleanup: Use 'in'.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.