fix(migration): correct serverid description

serverid is not generated at install time — it is a static integer
(default -1) that identifies a server in multi-PHP-server setups.
Separate it from secret/instanceid and accurately describe when it
matters and what to do with NC_serverid env overrides.

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/maintenance/migrating.rst

@@ -107,10 +107,14 @@ the new location. It is also assumed that the authentication method
       * ``tempdirectory`` — if set to a custom path, make sure that path exists and
         is writable on the new server.
 
-      The ``secret``, ``instanceid``, and ``serverid`` values are generated once at
-      install time and tied to all encrypted data, user sessions, and federation
-      identity. **Do not change or regenerate them.** They must be copied verbatim
-      from the original ``config.php``.
+      The ``secret`` and ``instanceid`` values are generated once at install time and
+      tied to all encrypted data and user sessions. **Do not change or regenerate them.**
+      They must be copied verbatim from the original ``config.php``.
+
+      If ``serverid`` is set, keep the same value. It identifies the server in
+      multi-PHP-server setups and must not change. If you override it per server via
+      the ``NC_serverid`` environment variable, configure the same override on the new
+      server.
 
       Leaving stale values (especially database credentials or ``datadirectory``)
       will cause startup errors or data corruption.