pp.c: Improve self-referential comment

pp.c: Improve self-referential comment

pp.c:pp_lc has this:

	    /* Here is where we would do context-sensitive actions.  See the
	     * commit message for this comment for why there isn't any */

If I try to look up the commit that added the comment, I get this:

commit 06b5486afd6f58eb7fdf8c5c8cdb8520a4c87f40
Author: Karl Williamson <public@khwilliamson.com>
Date:   Fri Nov 11 10:13:28 2011 -0700

    pp.c: White-space only

    This outdents and reflows comments as a result of the removal of a
    surrounding block

86510fb15 was the commit that added the comment, whose commit message
contains the explanation, so cite that directly.