help: suggest keyword search when no topic is found
When `hg help foobar` fails, it now suggests using `hg help --keyword foobar`
instead of printing a full page of basic commands.
This should greatly increases discoverability of the `hg help --keyword`
argument.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.