Document the user-level features of the Xref package

Document the user-level features of the Xref package

* doc/emacs/maintaining.texi (Maintaining): Add a list of
described features.
(Xref): New section, made out of thoroughly rewritten "Tags"
section.
(Find Identifiers, Looking Up Identifiers, Xref Commands)
(Identifier Search, List Identifiers): New subsections,
incorporating the old tags commands and the new xref commands.
(Tags Tables, Tag Syntax, Create Tags Table, Etags Regexps):
Section and subsections demoted to a lower level.
* doc/emacs/search.texi (Search):
* doc/emacs/windows.texi (Pop Up Window):
* doc/emacs/frames.texi (Creating Frames):
* doc/emacs/programs.texi (Imenu, Symbol Completion):
* doc/emacs/building.texi (Grep Searching):
* doc/emacs/dired.texi (Operating on Files):
* doc/emacs/glossary.texi (Glossary): All references to tags changed.