MAINT: Updates related to xref

[rossbar]

A few updates related to xref

• Use default alias mapping in make_xref test instead of custom one. From looking at git blame, it seems the original numpydoc_xref_aliases was replaced with DEFAULT_LINKS, but this change wasn't applied to the test suite. It's possible this was intentional, but I figured using the default dict made more sense.
• Updated info about the numpydoc_xref_aliases config val in install.rst. The description says the default is an empty dict, but the default is actually DEFAULT_LINKS. The current description also makes it seem like the default depends on the intersphinx configuration though it doesn't seem to AFAICT.
• Minor update to make_xref docstring

[codecov-commenter]

Codecov Report

Merging #292 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #292   +/-   ##
=======================================
  Coverage   93.10%   93.10%           
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  Files           7        7           
  Lines        1261     1261           
=======================================
  Hits         1174     1174           
  Misses         87       87           

[larsoner]

Thanks @rossbar