Document non-fast-forward requirement for topo bootstrap

Successive topo checkpoints aren't guaranteed to be in an ancestor-descendant relationship — topological order can interleave parallel branches — so the internal refs/gitsync/bootstrap/heads/ temp ref may receive non-fast-forward updates between batches. The temp ref is purely internal scaffolding (user-visible refs/heads and refs/tags only get a single fast-forward update at cutover), but targets that enforce receive.denyNonFastforwards across all refs will reject the temp-ref updates and fail the bootstrap.

Major hosts (GitHub, GitLab, Bitbucket, Cloudflare) don't enable this by default; the constraint only matters on hardened deployments. Document on the Strategy field, the BootstrapStrategy* SDK constants, and both CLI flag descriptions.