Update some doc string. Call `add-minor-mode'.

Update some doc string.  Call `add-minor-mode'.
(follow-windows-start-end-cache, follow-cache-command-list):
New variables.
(follow-cache-valid-p, follow-invalidate-cache): New functions.
(follow-windows-start-end, follow-post-command-hook,
follow-generic-filter): Cache support added.
(follow-avoid-tail-recenter): Problem with minibuffer-only
frames corrected.
(follow-windows-aligned-p): Minor change.