docs: remove :c:func: annotations from xarray.rst

Now that the build system automatically marks up function references, we
don't have to clutter the source files, so take it out.

[Some paragraphs could now benefit from refilling, but that was left out to
avoid obscuring the real changes.]
Acked-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>