(eval-when-compile): Require `help-macro'.

(eval-when-compile): Require `help-macro'.
(isearch-help-for-help-internal): Isearch specific Help screen
created by `make-help-screen'.
(isearch-help-map): New variable that binds "b", "k", "m" to
isearch-specific commands, and binds other keys to
isearch-other-control-char, thus allowing other Help keys to
exit isearch mode and execute their global definitions.
(isearch-help-for-help, isearch-describe-bindings)
(isearch-describe-key, isearch-describe-mode): New commands.
(isearch-mode-help): Define alias to isearch-describe-mode
and remove its old function definition.
(isearch-mode-map): Bind "\C-h" to isearch-help-map.
Remove old comments because this issue is settled now.
(isearch-forward): Describe three new Help commands in the docstring.
(isearch-mode): Remove \\{isearch-mode-map} from the docstring,
leave the single sentence at the first line of the docstring,
and prepend the word "function" before `isearch-forward'
to make the Help link to the function instead of variable.