Improve documentation of selection and navigation in *xref* buffers

Improve documentation of selection and navigation in *xref* buffers

* doc/emacs/maintaining.texi (Looking Up Identifiers): More
detailed description of 'xref-auto-jump-to-first-definition'.
Improve indexing.  Describe the use of 'next-error' and
'previous-error' in conjunction with the *xref* buffer.
(Identifier Search): More detailed description of
'xref-auto-jump-to-first-xref'.  Describe the use of 'next-error'
and 'previous-error'.