Documentation/sphinx: allow "functions" with no parameters

Documentation/sphinx: allow "functions" with no parameters

When kernel-doc:: specified in .rst document without explicit directives,
it outputs both comment and DOC: sections. If a DOC: section was explicitly
included in the same document it will be duplicated. For example, the
output generated for Documentation/core-api/idr.rst [1] has "IDA
description" in the "IDA usage" section and in the middle of the API
reference.

This patch enables using "functions" directive without parameters to output
all the documentation excluding DOC: sections.

[1] https://www.kernel.org/doc/html/v4.17/core-api/idr.html

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>