Improve documentation of new Xref options

Improve documentation of new Xref options

* lisp/progmodes/xref.el (xref-auto-jump-to-first-definition)
(xref-auto-jump-to-first-xref): More accurate description in the
doc strings.

* doc/emacs/maintaining.texi (Looking Up Identifiers)
(Identifier Search, List Identifiers, Project File Commands): Fix
the documentation of 'xref-auto-jump-to-first-definition' and
'xref-auto-jump-to-first-xref' to be more accurate.

* etc/NEWS: More accurate wording of the entry about
'xref-auto-jump-to-first-definition' and
'xref-auto-jump-to-first-xref'.