Documentation: /proc/loadavg: add 3 more field descriptions

Update contents of /proc/loadavg: add 3 more fields.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/fe55b139-bd03-4762-199b-83be873cf7dd@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>