Please document that tag deletion is difficult

[Glenn Morris]

Hi,

Just a minor doc bug.
If I understand the consequences of Bug #138802 correctly, it is basically impossible to ever delete a bzr tag.
Eg bzr://bzr.savannah.gnu.org/emacs/trunk has ~ 1000 tags, ~ 900 of which are not relevant (for historical reasons).
I deleted them, but since tag deletion does not propagate, this did not affect existing checkouts of the repository,
so the next time another developer committed something, all 900+ tags were re-added.

AFAICS, the only way to delete tags would be for everyone with write access to "simultaneously" delete them from their
copies; or to all check out a fresh copy. This is not feasible.

Please could you document in eg `bzr help tag' that tag deletion is impossible in many real-world cases.

Thanks.