Merge branch 'bk/sob-dco'

Merge branch 'bk/sob-dco'

Document that the meaning of a Signed-off-by trailer can vary from
project to project in the end-user documentation, and clarify what
it means to this project.

* bk/sob-dco:
  Documentation: stylistically normalize references to Signed-off-by:
  SubmittingPatches: clarify DCO is our --signoff rule
  Documentation: clarify and expand description of --signoff
  doc: preparatory clean-up of description on the sign-off option