[latexrefman] Add section on \baselineskip and \baselinestretch

[Karl Berry]

    I believe I added that section, although I have not gotten the email
    diff.

It was sent. I received it. And you are on the list. Went to spam maybe?
https://tug.org/pipermail/latexrefman-commits/

    thinking about it want to make a double spaced document, so that's what
    I made the synopsis. But perhaps others have a different view of the
    synopsis.

For a reference manual, true enough, I think the synopsis of
\baselineskip should not say to use setspace. I propose (and implemented :)
that the setspace example go in the setspace paragraph at the end.

\baselineskip is special in that, as you stated there, there is no
rational way to set it in LaTeX outside of font definition files (as far
as I know; it's extremely irritating). There could be a synopsis for
\linespread and \baselinestretch, but what is there seemed fine to
me, so I didn't go further.  --thanks for all, karl.