Add docs on upgrading from a very old version (#16951)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/16951.doc

@@ -0,0 +1 @@
+Add docs on upgrading from a very old version.

docs/upgrade.md

@@ -97,6 +97,26 @@ v1.61.0.
 
 <!-- REPLACE_WITH_SCHEMA_VERSIONS -->
 
+## Upgrading from a very old version
+
+You need to read all of the upgrade notes for each version between your current
+version and the latest so that you can update your dependencies, environment,
+config files, etc. if necessary. But you do not need to perform an
+upgrade to each individual version that was missed.
+
+We do not have a list of which versions must be installed. Instead, we recommend
+that you upgrade through each incompatible database schema version, which would
+give you the ability to roll back the maximum number of versions should anything
+go wrong. See [Rolling back to older versions](#rolling-back-to-older-versions)
+above.
+
+Additionally, new versions of Synapse will occasionally run database migrations
+and background updates to update the database. Synapse will not start until
+database migrations are complete. You should wait until background updates from
+each upgrade are complete before moving on to the next upgrade, to avoid
+stacking them up. You can monitor the currently running background updates with
+[the Admin API](usage/administration/admin_api/background_updates.html#status).
+
 # Upgrading to v1.100.0
 
 ## Minimum supported Rust version

docs/usage/administration/admin_faq.md

@@ -120,6 +120,11 @@ for file in $source_directory/*; do
 done
 ```
 
+How do I upgrade from a very old version of Synapse to the latest?
+---
+See [this](../../upgrade.html#upgrading-from-a-very-old-version) section in the
+upgrade docs.
+
 Manually resetting passwords
 ---
 Users can reset their password through their client. Alternatively, a server admin