Improve documentation of bidi in ELisp manual.

Improve documentation of bidi in ELisp manual.

 doc/lispref/nonascii.texi (Character Properties): Document use of
 `bidi-class' and `mirroring' properties as part of reordering.
 Provide cross-references to "Bidirectional Display".
 doc/lispref/display.texi (Bidirectional Display): Document the pitfalls of
 concatenating strings with bidirectional content, with possible
 solutions.  Document string-mark-left-to-right.  Mention paragraph
 direction in modes that inherit from prog-mode.  Document use of
 `bidi-class' and `mirroring' properties as part of reordering.
 etc/NEWS: Mark string-mark-left-to-right as documented.