bug#17862: 24.3; regexp-opt docstring is incorrect

[Stefan Monnier]

> I'd argue that this sole change requires me to special-case (or ..) form
> of rx macro for one arguments to avoid the unnecessary shy-grouping
> there, but the message of said commit contains no information about the
> background of that change, so it's hard to say if its benefits overweigh
> the inconveniences.

It makes the semantics of regexp-opt much simpler/cleaner since the
caller doesn't need to worry about the fact that the returned regexp
might need to be surrounded by additional grouping in order for things
like (concat (regexp-opt ..) "[ \t]+" ...) to work as intended.

A few redundant shy-group wrappers are a very small price to pay for
that, I think.  We could arrange to provide some way to avoid this
wrapper, I guess.


        Stefan