fix: highlight the required configuration and behavior of the fixer tools
The problem is that `hg help fix` didn't mention *how* to configure the tools,
and while I knew that `{rootpath}` existed in the configuration, I missed that
the tools require reading content from stdin. (I configured `gofmt` to use
`{rootpath}`, and that had the effect of squashing all changes in a file at `.`
into the first commit and emptying that content from its descendants.)
Basically all this does is put a pointer in the default (command level) help to
the extension level help that mentions the configuration, and moves the
extension level help that documents reading from stdin and writing to stdout to
the top to give it more prominence. The last sentence is adjusted a bit to
reflect the new location.
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.