Clarify the comment about setting the PYTHON variable for the Doc Makefile.

Doc/tools/sphinx-build.py

@@ -13,9 +13,10 @@ if __name__ == '__main__':
 
     if sys.version_info[:3] < (2, 5, 0):
         print >>sys.stderr, """\
-Error: Sphinx needs to be executed with Python 2.5 or newer.
+Error: Sphinx needs to be executed with Python 2.5 or newer
 (If you run this from the Makefile, you can set the PYTHON variable
-to the path of an alternative interpreter executable.)
+to the path of an alternative interpreter executable, e.g.,
+``make html PYTHON=python2.5``).
 """
         sys.exit(1)