Improve documentation of regexp search for categories.

Improve documentation of regexp search for categories.

 doc/lispref/searching.texi (Regexp Backslash): Document how to display
 existing categories.  Mention the possibility of adding
 categories, and add an xref to where this is described.  Add an
 index entry.
 doc/lispref/syntax.texi (Categories): Add an example of defining a new
 category and category table.