docs/boot-time-mm: fix kernel-doc directive for including all but DOC:

There were several rounds of the patches that enabled "functions" directive
with no parameters in kerneldoc.py to allow including all the kernel-doc
comments except the DOC: sections.

Yet, the boot-time-mm.rst sneaked in with the older version of that
directive and was not updated. Update it now.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>