Improve documentation of search functions

Improve documentation of search functions

Make the documentation of the search functions more accurate,
complete, and uniform; in particular, extend the description of
the effect when the 'count' parameter is a negative number to all
of these functions.

* src/search.c (Fsearch_backward, Fsearch_forward)
(Fre_search_backward, Fre_search_forward)
(Fposix_search_backward, Fposix_search_forward):
* lisp/isearch.el (word-search-backward, word-search-forward)
(word-search-backward-lax, word-search-forward-lax): Improve doc
strings as described above.

* doc/lispref/searching.texi (String Search, Regexp Search)
(POSIX Regexps): Use 'count' instead of 'repeat' as the name of
the fourth parameter of the *-search-{forward,backward} functions
and improve documentation as described above.