docs: Add explicit steps to verify FTS indexes after upgrading to 3.0.

The OS upgrade paths which go through 2.1 do not call
`upgrade-zulip-stage-2` with `--audit-fts-indexes` because that flag
was added in 3.0.

Add an explicit step to do this audit after the 3.0 upgrade.  Stating
it as another command to run, rather than attempting to tell them
to add it to the `upgrade-zulip` call that we're linking to seems
easiest, since that does not dictate if they should upgrade to a
release or from the tip of git.

We do not include a step describing this for the Trusty -> Xenial
upgrade, because the last step already chains into Xenial -> Bionic,
which itself describes auditing the indexes.

Fixes #15877.

docs/production/upgrade-or-modify.md

@@ -295,6 +295,12 @@ instructions for other supported platforms.
 6. [Upgrade to the latest Zulip release](#upgrading-to-a-release), now
    that your server is running a supported operating system.
 
+7. As root, finish by verifying the contents of the full-text indexes:
+
+    ```
+    /home/zulip/deployments/current/manage.py --audit-fts-indexes
+    ```
+
 ### Upgrading from Ubuntu 14.04 Trusty to 16.04 Xenial
 
 1. Upgrade your server to the latest Zulip `2.0.x` release.  You can
@@ -388,6 +394,12 @@ instructions for other supported platforms.
 6. [Upgrade to the latest Zulip release](#upgrading-to-a-release), now
    that your server is running a supported operating system.
 
+7. As root, finish by verifying the contents of the full-text indexes:
+
+    ```
+    /home/zulip/deployments/current/manage.py --audit-fts-indexes
+    ```
+
 ## Upgrading PostgreSQL
 
 Starting with Zulip 3.0, we use the latest available version of