How to cross-link to another document?

[Matthew Wilcox <willy@infradead.org>]

I've tried several variants of this without success.  What's the
correct incantation?

+++ b/Documentation/core-api/idr.rst
@@ -17,6 +17,8 @@ solution to the problem to avoid everybody inventing their own.  The IDR
 provides the ability to map an ID to a pointer, while the IDA provides
 only ID allocation, and as a result is much more memory-efficient.

+The IDR interface is deprecated; please use the _`XArray` instead.
+
 IDR usage
 =========

+++ b/Documentation/core-api/xarray.rst
@@ -1,4 +1,5 @@
 .. SPDX-License-Identifier: GPL-2.0+
+.. XArray_

 ======
 XArray

I first tried "named reference, reStructuredText_;"
and then "_`inline internal target`" and I've tried both prefixing and
suffixing both destination and target with an underscore.  Nothing seems
to work and sometimes I get error messages telling me it won't work,
and sometimes I just get non-hyperlinks.

Do I have to do something like

	:ref:`Documentation/core-api/xarray.rst <XArray>`
?  That would seem unnecessarily verbose.