docs: Don't push Sphinx upgrades quite so readily

The sphinx-pre-install script will put out a verbose message recommending
an upgrade for anybody running less than 2.4.4 - which was only released in
March.  So *everybody* will see that warning at this point.  Let's only
warn if the user is below our generally recommended version (1.7.9
currently).

It might be good to put out a warning if people are explicitly making PDF
files, but would need to be done in a different place and relatively few
people do that.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>