exthelper: improve docs to indicate what module vars are needed
I recently tried creating an extension "from scratch" using exthelper, and it
wasn't obvious that you needed these. I believe that a careful reading of one of
the comments would tell you that they were required, but it's easy to miss and
having the examples be "complete" is helpful.
Differential Revision: https://phab.mercurial-scm.org/D10295
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif