config.txt: Document help.htmlpath config parameter

Signed-off-by: Sebastian Staudt <koraktor@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'ss/help-htmlpath-config-doc' into next

Add missing doc.

* ss/help-htmlpath-config-doc:
  config.txt: Document help.htmlpath config parameter