(Word Search): Replace `C-s RET C-w' with `M-s w RET'

(Word Search): Replace `C-s RET C-w' with `M-s w RET'
as a key binding to start a forward nonincremental word search.
Replace `C-r RET C-w' with `M-s w C-r RET' as a key binding to start
a backward nonincremental word search.  Add index for `M-s w'
`isearch-forward-word'.
(Regexp Search): Add a short summary of regexp key commands like
in the node "Basic Isearch".
(Other Repeating Search): Fix typo.