Documentation: add reference from dynamic debug to loglevel kernel params

This is useful information for somebody who has managed to dig into
enabling debug output, but is wondering why there is no such output
appearing on the console.

Signed-off-by: Daniel Lublin <daniel@lublin.se>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4633bdb82c1c7c014d79840887878624a55c59f8.1720533043.git.daniel@lublin.se