* lisp/isearch.el (isearch-word): Add docstring.

* lisp/isearch.el (isearch-word): Add docstring.
(isearch-occur, isearch-search-and-update): If `isearch-word' is
a function, call it to get the regexp.
(isearch-message-prefix): If `isearch-word' holds a symbol, use its
property `isearch-message-prefix' instead of the string "word ".
(isearch-search-fun-default): For the case of `isearch-word',
return a lambda that calls re-search-forward/re-search-backward
with a regexp returned by `word-search-regexp' or by the function
in `isearch-word'.