Added a tip about --layer param of ogrinspect for databases

Refs #20260.

docs/ref/contrib/gis/commands.txt

@@ -47,7 +47,8 @@ of using ``ogrinspect`` :ref:`in the tutorial <ogrinspect-intro>`.
    The key for specifying which layer in the OGR
    :class:`~django.contrib.gis.gdal.DataSource` source to use.
    Defaults to 0 (the first layer). May be an integer or a string identifier
-   for the :class:`~django.contrib.gis.gdal.Layer`.
+   for the :class:`~django.contrib.gis.gdal.Layer`. When inspecting databases,
+   ``layer`` is generally the table name you want to inspect.
 
 .. django-admin-option:: --mapping