Update Syntax chapter of Lisp manual.

Update Syntax chapter of Lisp manual.

* doc/lispref/syntax.texi (Syntax Tables, Syntax Descriptors)
(Syntax Table Functions): Copyedits.
(Syntax Basics): Don't repeat the material in the preceding node.
(Syntax Class Table): Use a table.
(Syntax Properties): Document syntax-propertize-function and
syntax-propertize-extend-region-functions.
(Motion via Parsing): Fix indentation.
(Parser State): Update for the new "c" comment style.  Fix
description of item 7 (comment style).

* doc/lispref/modes.texi (Syntactic Font Lock): Add xref to Syntactic Font Lock node.