In Emacs, how I can I instruct isearch-forward
to search not from the location of the point, but from the beginning of the file?
The simplest option is to just move the point to the beginning of file with M-< before invoking isearch. Or, you can write a command that does it for you:
(defun isearch-from-buffer-start ()
(interactive)
(goto-char (point-min))
(isearch-forward))
(global-set-key [(control s)] 'isearch-from-buffer-start)