Rework the EXAMPLES section to be a bit clearer.
- Add an example of using etcupdate diff.
- Create a subsection on bootstrapping that is below the simple
  examples.  This should make it clearer that 'etcupdate extract' is
  a one-time operation and not part of the common workflow.  It also
  adds more suggestions on when bootstrapping is needed and additional
  steps to make future merges simpler.

Reviewed by:	adrian
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273834 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f