bug#37659: rx additions: anychar, unmatchable, unordered-or

[Drew Adams]

> > Is there an identifiable subset of rx features ... that map
> > (even if not one-to-one) to regexp syntax components?
> 
> C-h f rx
> 
> (syntax SYNTAX)  Match a character with syntax SYNTAX, being one of:
>   whitespace, punctuation, word, symbol, open-parenthesis,
>   close-parenthesis, expression-prefix, string-quote,
>   paired-delimiter, escape, character-quote, comment-start,
>   comment-end, string-delimiter, comment-delimiter

Yes, that's fine for char syntax classes.  It's
good that their correspondences are listed.

But for, say, `line-start' (aka `bol') there is a
verbal description but no mention of the Elisp
regexp syntax that corresponds:

  ‘line-start’, ‘bol’
     matches the empty string, but only at the
     beginning of a line in the text being matched

That's the kind of thing I was suggesting.  It would
be helpful, I think, to mention (somewhere) that the
regexp syntax for this is "^".

Same thing for the other constructs (`string-start'
and all the rest).

I'm using Emacs 26.3.  I didn't find anything beyond
the doc string - nothing in the Emacs or Elisp manual
(which is OK).