feat(devmanual): Clarify behavior of missing replacement indexes

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

Author: ChristophWurst

developer_manual/basics/storage/migrations.rst

@@ -198,6 +198,8 @@ Replacing indices
 
 Similar to adding an index to an existing table, it could be necessary to replace one or more indices with a new one. To avoid a gap between dropping the old indices in a migration and adding the new one through ``AddMissingIndicesEvent``, it is possible to do both at once in ``AddMissingIndicesEvent``.
 
+If none of the previous indices are found, e.g. because they were optional and not created yet, the replacement index will be treated as *missing index*.
+
 .. note:: Make sure to not use the same index name for the new index as for old indices.
 
 .. code-block:: php