Documentation: proc.rst: add more about the 6 fields in loadavg

Address Jon's feedback on the previous patch by adding info about
field separators in the /proc/loadavg file.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20210222034729.22350-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>