Add some usage info -fdebug-prefix-map= docs.

	gcc/
	* doc/invoke.texi (-fdebug-prefix-map): Expand documentation.

From-SVN: r254121

gcc/ChangeLog

+2017-10-26  Jim Wilson  <wilson@tuliptree.org>
+
+	* doc/invoke.texi (-fdebug-prefix-map): Expand documentation.
+
 2017-10-26  Tom de Vries  <tom@codesourcery.com>
 
 	PR tree-optimization/82707

gcc/doc/invoke.texi

@@ -6982,7 +6982,12 @@ link processing time.  Merging is enabled by default.
 @item -fdebug-prefix-map=@var{old}=@var{new}
 @opindex fdebug-prefix-map
 When compiling files in directory @file{@var{old}}, record debugging
-information describing them as in @file{@var{new}} instead.
+information describing them as in @file{@var{new}} instead.  This can be
+used to replace a build time path with an install time path in the debug info.
+It can also be used to change an absolute path to a relative path by using
+@file{.} for @var{new}.  This can give more reproducible builds, which are
+location independent, but may require an extra command to tell GDB where to
+find the source files.
 
 @item -fvar-tracking
 @opindex fvar-tracking