Here is how to make C-h show help from within IncrementalSearch:
(define-key isearch-mode-map [(control h)] 'isearch-mode-help)
And if you want to read the help too you can allow scrolling by setting isearch-allow-scroll.
Other than that, use C-h f isearch-mode…
‘C-h k C-s’…
Library Lisp:isearch+.el makes this binding, but it also redefines
‘isearch-mode-help’ so that it: