docs: Clarify the usage and sign-off requirements for Co-developed-by

The documentation for Co-developed-by is a bit light on details, e.g. it
doesn't explicitly state that:

  - Multiple Co-developed-by tags are perfectly acceptable
  - Co-developed-by and Signed-off-by must be paired together
  - SOB ordering should still follow standard sign-off procedure

Lack of explicit direction has resulted in developers taking a variety
of approaches, often lacking any intent whatsoever, e.g. scattering SOBs
willy-nilly, collecting them all at the end or the beginning, etc...
Tweak the wording to make it clear that multiple co-authors are allowed,
and document the expectation that standard sign-off procedures are to
be followed.

The use of "original author" has also led to confusion as many patches
don't have just one "original" author, e.g. when multiple developers
are involved from the genesis of the patch.  Remove all usage of
"original" and instead call out that Co-developed-by is simply a way to
provide attribution in addition to the From tag, i.e. neither tag is
intended to imply anything with regard to who did what.

Provide examples to (hopefully) eliminate any ambiguity.

Cc: Tobin C. Harding <me@tobin.cc>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Niklas Cassel <niklas.cassel@linaro.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>