ADDED: user option to control predicate documentation composition

ADDED: user option to control predicate documentation composition

* sweeprolog.el
(sweeprolog-insert-pldoc-for-predicate): remove in favor of...
(sweeprolog-insert-predicate-documentation): new function.
 (sweeprolog-format-term-with-holes): new utility
function, used by...
(sweeprolog-read-predicate-documentation-with-holes)
(sweeprolog-read-predicate-documentation-default-function): new
functions, used for...
(sweeprolog-read-predicate-documentation-function): new user option,
used by...
(sweeprolog-read-predicate-documentation): new function, used by...
(sweeprolog-document-predicate-at-point): refactor.