[patch] Improve docs on running libstdc++ tests

[Jonathan Wakely <jwakely at redhat dot com>]

This came up on IRC today. Editing testsuite_files isn't the only way
to run a subset of tests.

Committed to trunk.

commit 97f1afff492953efa83e07fdbf66af2b77a5ee89
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Apr 10 20:58:54 2015 +0100

    	* doc/xml/manual/test.xml: Improve documentation on running a subset
    	of tests.
    	* doc/html/manual/test.html: Regenerate.

diff --git a/libstdc++-v3/doc/xml/manual/test.xml b/libstdc++-v3/doc/xml/manual/test.xml
index d3d2264..456c48b 100644
--- a/libstdc++-v3/doc/xml/manual/test.xml
+++ b/libstdc++-v3/doc/xml/manual/test.xml
@@ -279,12 +279,26 @@ make check-target-libstdc++-v3 RUNTESTFLAGS="-v -v"
 </programlisting>
 
     <para>
-      To run a subset of the library tests, you will need to generate
-      the <emphasis>testsuite_files</emphasis> file by running
+      To run a subset of the library tests, you can either generate the
+      <emphasis>testsuite_files</emphasis> file (described below) by running
       <command>make testsuite_files</command> in the
-      <emphasis>libbuilddir/testsuite</emphasis> directory, described
-      below.  Edit the file to remove the tests you don't want and
-      then run the testsuite as normal.
+      <emphasis>libbuilddir/testsuite</emphasis> directory, then edit the
+      file to remove the tests you don't want and then run the testsuite as
+      normal, or you can specify a testsuite and a subset of tests in the
+      RUNTESTFLAGS variable.
+    </para>
+
+    <para>
+      For example, to run only the tests for containers you could use:
+    </para>
+
+<programlisting>
+make check-target-libstdc++-v3 RUNTESTFLAGS="conformance.exp=23_containers/*"
+</programlisting>
+
+    <para>
+      When combining this with other options in RUNTESTFLAGS the
+      <emphasis>testsuite.exp=testfiles</emphasis> options must come first.
     </para>
 
     <para>