Revert "Simplify the instructions for updating Tor Browser."

This reverts commit b886a25c.

The changes introduced by b886a25c certainly make things easier for the case when
we're importing the tarballs into our git-annex, but it makes this piece of doc
unusable for other usecases, most notably:

 * quickly testing a new stable or alpha Tor Browser _before_ uploading hundreds
   of MiB
 * for contributors who don't have write access to our git-annex thingie, but
   still want to work on things that involve including a custom, experimental,
   not-released-yet or whatnot Tor Browser in the ISO.