Consistency for xref-show-{xrefs,definitions}-function

[Gabriel do Nascimento Ribeiro]

I noticed that the options 'xref-show-xrefs-function' and
'xref-show-definitions-function' are not symmetrical.

The defcustom for 'xref-show-xrefs-function' does not have type choices
as 'xref-show-definitions-function' has. Furthermore, it's quite strange
to configure it as follow:

(setq xref-show-xrefs-function
      'xref-show-definitions-completing-read)

I suggest to create similar functions for 'xref-show-xrefs-function',
which at this moment can be just aliases but can be independent
functions in the future, if necessary:
  - xref-show-xrefs-buffer
  - xref-show-xrefs-buffer-at-bottom
  - xref-show-xrefs-completing-read

Also, the docstrings of both options could be improved a little bit to
give more information about the default choices.

I can submit a patch if these suggestions are accepted.

Regards,
Gabriel