Add docs on upgrading from a very old version

changelog.d/16951.doc

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

docs/upgrade.md

@@ -97,6 +97,28 @@ 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 anything has changed. 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. But, 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. Say you are upgrading from 1.48.0, you could update to v1.51.0 (and still
+be able to roll back to v1.48.0, or all the way back to v1.39.0), then v1.57.1
+(and still roll back to v1.49.0), etc.
+
+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
+[this 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