The rx notation is useful and complex enough to merit inclusion in the manual.

Right now, it's mainly described in the `rx' doc string, which is fairly well-written but quite long and a bit unstructured. Describing it in the manual permits a different pace and style of exposition, the inclusion of examples and related information, structured into separate sections with cross-references.

Proposed patch attached. It covers all rx features, functions, macros, including the pcase pattern, and a mention of the corresponding string regexp constructs.

The existing `rx' doc string can be left unchanged, or reduced to something more concise, perhaps without a description of the entire rx language but with a manual reference. Suggestions are welcome.