replay: improve --contained and add to doc

replay: improve --contained and add to doc

There is no documentation for `--contained`.

Start by copying the text from `replay_options` in `builtin/
replay.c`. But some people think that the existing text is a
bit unclear; what does it mean for a branch to be contained
in a revision range? Let’s include the implied commits here:
the branches that point at commits in the range.

Also use “update” instead of “advance”. “Update” is the verb
commonly used in this context.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>