narrow: remove hack to write narrowspec to shared .hg directory
AFAIK, we no longer need it since the narrowspec file was move to the
store directory in 576eef1ab43d, "narrow: move .hg/narrowspec to
.hg/store/narrowspec."
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.