lisp/so-long.el: Improve support for major mode hooks

lisp/so-long.el: Improve support for major mode hooks

* lisp/so-long.el (so-long-remember-all, so-long-disable-minor-modes)
(so-long-override-variables): Store and use the `so-long-minor-modes'
and `so-long-variable-overrides' values seen by the original major
mode, so that buffer-local changes made in the major mode hook will be
respected.

Add documentation of this and other major mode hook usage.