Fix the documentation of various aspects of adding Xref history

Fix the documentation of various aspects of adding Xref history

* lisp/progmodes/xref.el (xref-marker-ring-length)
(xref-set-marker-ring-length):
* lisp/progmodes/etags.el (tags-location-ring-length)
(find-tag-marker-ring): Add doc strings saying the variables are
unused.

* etc/NEWS: Enhance the description of the change which made Xref
marker stack unlimited in its length.

* doc/emacs/maintaining.texi (Looking Up Identifiers): Add back
text lost when xref forward history was added in bug#38797.
Explain the difference between 'C-M-,' and 'M-.'.  Improve
wording (Bug#62229)