ada: Add new Mingw task priority mapping

ada: Add new Mingw task priority mapping

This patch adds a new mapping (Non_FIFO_Underlying_Priorities) for dynamically
setting task priorities in Windows when pragma Task_Dispatching_Policy
(FIFO_Within_Priorities) is not present. Additionally, it documents the
requirement to specify the pragma in order to use Set_Priority in the general
case.

gcc/ada/

	* doc/gnat_ugn/platform_specific_information.rst: Add note about
	different priority level granularities under different policies in
	Windows and move POSIX related info into new section.
	* libgnarl/s-taprop.ads: Add note about Task_Dispatching_Policy.
	* libgnarl/s-taprop__mingw.adb:
	(Set_Priority): Add use of Non_FIFO_Underlying_Priorities.
	* libgnat/system-mingw.ads: Add documentation for modifying
	priority mappings and add alternative mapping
	Non_FIFO_Underlying_Priorities.
	* gnat_ugn.texi: Regenerate.