* lisp/isearch.el: Improve 'isearch-allow-motion' feature (bug#50466)

* lisp/isearch.el: Improve 'isearch-allow-motion' feature (bug#50466)

* lisp/isearch.el: Add recenter to 'isearch-motion' property of
'end-of-buffer' to maximize the number of search hits on the screen.
In 'isearch-motion' property of 'scroll-up-command' use 'recenter 0'
for the first line of the screen.
(isearch-beginning-of-buffer)
(isearch-end-of-buffer): Sync logic from 'isearch-allow-motion' part
of isearch-pre-command-hook.  Direct users to isearch-allow-motion
in the docstrings.
(isearch-pre-command-hook): Don't override shifted 'isearch-yank-on-move'
in 'isearch-allow-motion'.