Enabling matrix_hookshot_encryption_enabled configures the bridge side
correctly (registration flags, Valkey cache), but hookshot's encryption
also needs the homeserver to support and enable MSC2409 and MSC3202,
which are typically experimental features disabled by default. Nothing
documented that, so the resulting setup silently did not work.

Reference the MSCs in the variable's comment, and show the exact Synapse
settings in the hookshot documentation page. The playbook deliberately
does not auto-enable them: they are experimental homeserver-wide
features, and flipping them from a bridge toggle would be surprising.

Fixes #3861

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>