[latexrefman] Font styles & sizes as environments

[Karl Berry]

    display math mode could be obtained with $$...$$ but
    (fortunately) it isn't mentioned in the reference manual.

I gather $$ isn't mentioned in Lamport or Companion (I don't have the
books, so I'm not completely sure, but I guess this is the case), so
ordinarily I wouldn't feel any need to add it to latex2e.texi, since
it's not officially supported.

But since $$ does in fact work, and is such a fundamental bit of TeX
syntax, I think it is fine that we mention it, along with an admonition
not to use it. Jim wrote some cautionary words about it already. I just
made some small changes.

Not that it matters, but BTW, I find myself unconvinced by all the
arguments I've seen against it, except fleqn. Sure, the resulting
vspacing is imperfect in some cases. That is so with \[...\] too;
.6\linewidth can't possibly always be the perfect amount of space.
Sometimes you just have to fiddle ... --thanks, karl.