docs/idr: use empty "functions" directive

to avoid duplication of DOC: sections in the middle of the API reference.
Signed-off-by: NMike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: NMatthew Wilcox <willy@infradead.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/idr.rst

@@ -76,4 +76,6 @@ Functions and structures
 ========================
 
 .. kernel-doc:: include/linux/idr.h
+   :functions:
 .. kernel-doc:: lib/idr.c
+   :functions: