Documentation: build redo-jch.sh from master..jch

Documentation: build redo-jch.sh from master..jch

Rebuilding the 'jch' branch begins by reintegrating any topics between
'master' and 'jch', not 'master' and 'seen'.

In the maintainer guide, the documentation isn't quite right, since the
initial input to Meta/Reintegrate is "master..seen", not "master..jch".
This can lead to confusing results when generating the Meta/redo-jch.sh
script for the first time.

Additionally, rebuilding 'jch' takes place in two steps. First, running
the script up to the first "### match next" cut-line, and then comparing
the result with what's on 'next' (i.e. with "git diff jch next"). Then,
the remaining set of topics get merged down to 'jch' (which aren't on
'next') by running the entire "redo-jch.sh" script.

Clarify the documentation to reflect this.

Signed-off-by: Taylor Blau <me@ttaylorr.com>