Documentation/submitting-patches: Extend commit message layout description

Add more blurb about the level of detail that should be contained in a
patch's commit message. Extend and make more explicit what text should
be added under the --- line. Extend examples and split into more easily
palatable paragraphs.

This has been partially 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>
Reviewed-by: Robert Richter <rrichter@amd.com>
Link: https://lore.kernel.org/r/20210215141949.GB21734@zn.tnic


[jc: Tweaked "example subjects" wording]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>