Fix Isearch prompt when invoked with an argument

Fix Isearch prompt when invoked with an argument

* lisp/isearch.el (isearch--describe-regexp-mode): With
`search-default-mode' set to nil, if user does C-u C-s, the minibuffer
now displays "Regexp I-search: " again.  But if the user has set
`search-default-mode' to t, and then does C-s, the minibuffer now
displays "I-search: " because the default search mode is now regexp
mode.  Comments have been added to explain the priority of conditions
in the `cond' form.  (Bug#22991)