doc/INSTALL: Remove rst2man reference and other updates

doc/INSTALL: Remove rst2man reference and other updates

The rst2man target was removed in 9d9a700 (doc: build man pages at
build time; introduce HAVE_SPHINX, HAVE_RST2MAN, 2014-03-13), but a
reference in the install docs slipped through.  While I was removing
that reference, I also:

* Converted doc/INSTALL to reStructuredText, so I can link to Sphinx
  and Docutils directly.  Not everyone has access to Debian's
  python-docutils, so it's better to be genric here.
* Converted from an unordered list to paragraphs, because I think it
  flows better.
* Dropped the rst2man no-automatic-install caveat.  I don't think this
  applies to the current code, although I haven't tried to track down
  a commit that adds the automatic-install support.  Anyhow,

    $ make HAVE_SPHINX=0 RST2MAN=/usr/bin/rst2man.py DESTDIR=/tmp/ install-man

  works for me.