[Ada] Clarify meaning of local pragma Warnings Off without On

[Ada] Clarify meaning of local pragma Warnings Off without On

A local use of pragma Warnings Off to suppress specific messages, when
not followed by a matching pragma Warnings On, extends until the end of
the file.

2018-05-23  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Clarify meaning of
	local pragma Warnings Off without On.
	* gnat_rm.texi: Regenerate.

From-SVN: r260589