new *Help* argument highlighting

[Miles Bader]

I've just seen the effect of the argument highlighting in
describe-function *Help* buffers, and it seems like _way_ too much.
It's nice to have some highlighting for the args, but I think
`font-lock-variable-name-face' is the wrong thing to use.  In an
otherwise monochrome help-buffer, that makes the args stand out like
lasers, and distracts attention from everything else (a typical source
buffer using font-lock is a neon fruit salad anyway, so that face is
appropriate there).

I tried changing the `help-argument-name' face to be just italic
instead, and that seemed a bit better (after all in documentation
italics is often used for arguments), but since the args are uppercased,
it looked slightly odd -- uppercase italics are generally a bit weird.
I ended up thinking `the arguments are _already_ in uppercase, which is
pretty noticeable, why is necessary to highlight them further at all?'

-Miles
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