[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

gcc/ada/ChangeLog

+2018-05-23  Yannick Moy  <moy@adacore.com>
+
+	* doc/gnat_rm/implementation_defined_pragmas.rst: Clarify meaning of
+	local pragma Warnings Off without On.
+	* gnat_rm.texi: Regenerate.
+
 2018-05-23  Olivier Hainque  <hainque@adacore.com>
 
 	* libgnat/g-excact.ads (Is_Foreign_Exception): New predicate.

gcc/ada/doc/gnat_rm/implementation_defined_pragmas.rst

@@ -7456,6 +7456,10 @@ In this usage, the pattern string must match in the Off and On
 pragmas, and (if *-gnatw.w* is given) at least one matching
 warning must be suppressed.
 
+Note: if the ON form is not found, then the effect of the OFF form extends
+until the end of the file (pragma Warnings is purely textual, so its effect
+does not stop at the end of the enclosing scope).
+
 Note: to write a string that will match any warning, use the string
 ``"***"``. It will not work to use a single asterisk or two
 asterisks since this looks like an operator name. This form with three

gcc/ada/gnat_rm.texi

@@ -8893,6 +8893,10 @@ In this usage, the pattern string must match in the Off and On
 pragmas, and (if @emph{-gnatw.w} is given) at least one matching
 warning must be suppressed.
 
+Note: if the ON form is not found, then the effect of the OFF form extends
+until the end of the file (pragma Warnings is purely textual, so its effect
+does not stop at the end of the enclosing scope).
+
 Note: to write a string that will match any warning, use the string
 @code{"***"}. It will not work to use a single asterisk or two
 asterisks since this looks like an operator name. This form with three