Update the documentation of overlays (bug#59996)

Update the documentation of overlays (bug#59996)

* src/buffer.c (Foverlay_recenter, Foverlay_lists): Update the doc
strings.

* lisp/subr.el (copy-overlay): Update comment.

* doc/lispref/internals.texi (Buffer Internals): Remove buffer
fields relevant to the old implementation; add the new interval
tree field.
* doc/lispref/display.texi (Overlays, Managing Overlays): Update
text to be consistent with the new implementation of overlays.
(Managing Overlays): Remove documentation of 'overlay-recenter'.

* etc/NEWS: Mention incompatible aspects of overlay
reimplementation.