[latexrefman] Added section on \stretch

[Karl Berry]

    (I understand Lamport's book to say that the argument must be
    integral, but testing seems to me to show that it can be any
    number.)

Yes, it can be any number:

ltspace.dtx:1310:\def\stretch#1{\z@ \@plus #1fill\relax}

i.e., the expansion is "0pt plus #1fill", and certainly any <number> is
allowable as the factor for fil(l)(l) value

Aside: personally, I find LaTeX's obscurantism less than helpful. I
often have to use actual fil, fill, filll units. There's a reason why
they exist.  Anyway, the new section looks good to me, FWIW. --best, karl.