docs: adjust the technical overview for the rename `pu` -> `seen`

docs: adjust the technical overview for the rename `pu` -> `seen`

This patch tries to rewrite history a bit: the mail contents that have
been added to Git's source code are actually fixed, we cannot change
them in hindsight.

But as the `pu` branch _was_ renamed, and as the documents were added to
Git's source code not so much as historical record, but to describe the
status quo, let's pretend that we have a time machine and adjust the
provided information accordingly.

Where appropriate, quotes were added for readability.

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