docs/idr: use empty "functions" directive

to avoid duplication of DOC: sections in the middle of the API reference.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>