xarray: Add documentation

This is documentation on how to use the XArray, not details about its
internal implementation.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Josef Bacik <jbacik@fb.com>

Documentation/core-api/index.rst

@@ -21,6 +21,7 @@ Core utilities
    local_ops
    workqueue
    genericirq
+   xarray
    flexible-arrays
    librs
    genalloc