doc: translate from :hg:`help config` to a valid link to hgrc.5.html Before this patch, ":hg:`help config`" in online help text is translated to a link to "hg.1.html#config" in HTML, even though actual "hg help config" shows not help for "hg config" command but "config" help topic, and all of current ":hg:`help config`" expects the latter. This patch translates from ":hg:`help config`" in online help text to a link to "hgrc.5.html" in HTML as expected. This patch also allows ":hg:`help -c COMMAND`" style to link "hg.1.html#COMMAND" for readability.