ignore: note debugignore on ignore man page

It took me a long time to find debugignore. I found the ignore man page
quickly. This change adds a debugging section to the ignore man page
letting people know there is a debug command.

mercurial/helptext/hgignore.txt

@@ -95,3 +95,9 b' Here is an example ignore file. ::'
   # switch to regexp syntax.
   syntax: regexp
   ^\.pc/
+
+Debugging
+=========
+
+Use the ``debugignore`` command to see if and why a file is ignored, or to
+see the combined ignore pattern. See :hg:`help debugignore` for details.