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>

Documentation/git-replay.adoc

@@ -42,6 +42,10 @@ The history is replayed on top of the <branch> and <branch> is updated to
 point at the tip of the resulting history. This is different from `--onto`,
 which uses the target only as a starting point without updating it.
 
+--contained::
+	Update all branches that point at commits in
+	<revision-range>. Requires `--onto`.
+
 --ref-action[=<mode>]::
 	Control how references are updated. The mode can be:
 +