Docs: correct the "active HEAD propagation needs symref-update" claim

Earlier docs framed active HEAD propagation as blocked on a future go-git change for the symref-update receive-pack capability. That capability doesn't exist in git's wire protocol — gitprotocol-pack.adoc defines receive-pack commands as create/delete/update only, and receive-pack's command parser rejects anything else. The symref-update that exists in git is in update-ref --stdin, a local CLI for the ref backend.

There is no portable wire-level mechanism for updating a remote symref, which is why git push --mirror doesn't either. Reframe the docs to match: detection is the ceiling for any git-protocol client, and manual reconciliation (provider REST API or git symbolic-ref over SSH) is the supported path, not a v1 placeholder.

Refs #45.