docs: proc.rst: comm: mention the included NUL

Indicate that the actual value will be one character less.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Link: https://lore.kernel.org/r/20240205154100.736499-1-mail@christoph.anton.mitterer.name


[jc: did s/null/NUL/ ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>