IDR: Note that the IDR API is deprecated

Some people read the documentation, perhaps.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Documentation/core-api/idr.rst

@@ -17,6 +17,9 @@ 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 :doc:`XArray <xarray>`
+instead.
+
 IDR usage
 =========