Replace :c:func: with func() as the previous usage is deprecated.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20200810183019.22170-1-puranjay12@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

This documentation was inadvertently released under the CC-BY-SA-4.0
license.  It was intended to be released under GPL-2.0 or later.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

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>

Move the idr kernel-doc to its own idr.rst file and add a few
paragraphs about how to use it.  Also add some more kernel-doc.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>