sourcebuild.texi: Document usage of line number 0 in verify compiler messages directives.

	* doc/sourcebuild.texi: Document usage of line number 0 in verify
	compiler messages directives.

From-SVN: r261493

gcc/ChangeLog

+2018-06-12  Jozef Lawrynowicz  <jozef.l@mittosystems.com>
+
+	* doc/sourcebuild.texi: Document usage of line number 0 in verify
+	compiler messages directives.
+
 2018-06-12  Matthew Fortune  <mfortune@gmail.com>
 
 	* config/mips/mips-cpus.def: New MIPS_CPU for i6500.

gcc/doc/sourcebuild.texi

@@ -1177,6 +1177,8 @@ conditions (which are the same as for @code{dg-skip-if}) are met.
 @end table
 
 @subsubsection Verify compiler messages
+Where @var{line} is an accepted argument for these commands, a value of @samp{0}
+can be used if there is no line associated with the message.
 
 @table @code
 @item @{ dg-error @var{regexp} [@var{comment} [@{ target/xfail @var{selector} @} [@var{line}] ]] @}