Improve documentation of search and replace commands

Improve documentation of search and replace commands

* doc/emacs/search.texi (Replacement and Lax Matches): Document
which commands are affected by 'replace-character-fold'.
(Lax Search): Add a cross reference to "Replacement and Lax
Matches".  Improve wording.  Fix lost extra whitespace.
(Search Customizations): Improve wording.  (Bug#22036)
See also comments in
http://lists.gnu.org/archive/html/emacs-devel/2015-11/msg02376.html.

* lisp/replace.el (query-replace, query-replace-regexp)
(query-replace-regexp-eval, replace-string, replace-regexp):
Mention 'replace-character-fold' in the doc strings.