Clarify needed branches in Tails Installer release documentation and update doc to rename

running
gbp import-orig \
—upstream-vcs-tag=“tails-installer_$NEW_UPSTREAM_VERSION” \
../tarballs/liveusb-creator_$NEW_UPSTREAM_VERSION+dfsg.orig.tar.gz

gives

Repository does not have branch ‘upstream/3.x+dfsg’ for upstream sources. If there is none see
file:///usr/share/doc/git-buildpackage/manual-html/gbp.import.html#GBP.IMPORT.CONVERT
on howto create it otherwise use —upstream-branch to specify it.
so I had to check out upstream/3.x+dfsg

our docs might want to have a step where we check out upstream/3.x+dfsg and merge origin

Parent Task: #8549 (closed)

Related issues

Related to #8555 (closed)

Original created by @u on 10425 (Redmine)

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