Documentation/submitting-patches: Document the SoB chain

Document what a chain of Signed-off-by's in a patch commit message
should mean, explicitly.

This has been carved out from a tip subsystem handbook patchset by
Thomas Gleixner:

  https://lkml.kernel.org/r/20181107171010.421878737@linutronix.de



and incorporates follow-on comments.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20201217183756.GE23634@zn.tnic


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