Add doc about bigints

Signed-off-by: Joas Schilling <coding@schilljs.com>

admin_manual/configuration_database/bigint_identifiers.rst

@@ -0,0 +1,21 @@
+==========================
+BigInt (64bit) identifiers
+==========================
+
+Since Nextcloud 13 big integers are used to store identifiers and auto-increment keys in the database.
+Because changing columns on huge tables can take quite a while (up to hours or days), the update from
+Nextcloud 12 or earlier did not perform this migration on the filecache and activity table.
+
+To make it easy to force the update on that tables too, we added a console command, which can be used
+to migrate the remaining columns to bigints.
+
+The command can safely be executed. It will show a success message when there is nothing to do::
+
+    sudo -u www-data ./occ db:convert-filecache-bigint
+    All tables already up to date!
+
+or otherwise ask for confirmation, before performing the heavy actions::
+
+    sudo -u www-data ./occ db:convert-filecache-bigint
+    This can take up to hours, depending on the number of files in your instance!
+    Continue with the conversion (y/n)? [n]

admin_manual/configuration_database/index.rst

@@ -5,6 +5,7 @@ Database Configuration
 .. toctree::
     :maxdepth: 2
 
-    db_conversion.rst
-    linux_database_configuration.rst
+    db_conversion
+    linux_database_configuration
     mysql_4byte_support
+    bigint_identifiers