Yet another doc improvement for search commands

Yet another doc improvement for search commands

* doc/emacs/search.texi (Word Search, Symbol Search)
(Regexp Search): Document commands that don't support lax
whitespace matching or character folding.
(Nonincremental Search): Mention the search commands that can be
invoked from the menu bar.

* lisp/isearch.el (isearch-define-mode-toggle-word)
(isearch-define-mode-toggle-symbol)
(isearch-define-mode-toggle-character-fold): Note in the doc
string that turning these on exits the regexp mode.
(isearch-forward-regexp, isearch-forward-word)
(isearch-forward-symbol, isearch-backward-regexp)
(word-search-backward, word-search-forward)
(word-search-backward-lax, word-search-forward-lax): State in the
doc string which commands don't support character folding and/or
lax-whitespace matching.