docs: adjust for the recent rename of `pu` to `seen`

docs: adjust for the recent rename of `pu` to `seen`

As of "What's cooking in git.git (Jun 2020, #04; Mon, 22)", there is no
longer any `pu` branch, but a `seen` branch.

While we technically do not even need to update the manual pages, it
makes sense to update them because they clearly talk about branches in
git.git.

Please note that in two instances, this patch not only updates the
branch name, but also the description "(proposed updates)".

Where appropriate, quotes have been added for readability.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>