doc: make the intent of sentence clearer

doc: make the intent of sentence clearer

By adding the word "just", which might have been accidentally omitted.

Adding the word "just" makes it clear that the point is to *not* do an
octopus merge simply because you *can* do it.  In other words, you
should have a reason for doing it beyond simply having two (seemingly)
independent commits that you need to merge into another branch, since
it's not always the best approach.

The previous sentence made it look more like it was trying to say that
you shouldn't do an octopus merge *because* you can do an octopus merge.
Although this interpretation doesn't make sense and the rest of the
paragraph makes the intended meaning clear, this adjustment should make
the intent of the sentence more immediately clear to the reader.

Signed-off-by: Kristoffer Haugsbakk <kristoffer.haugsbakk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>