Updates layer selection to allow optional default

- Modifies layer argument handling to permit no default layers appropriate.
- Update the help text for the layer parameter in the reverse command

src/nominatim_db/clicmd/api.py

@@ -104,7 +104,7 @@ def _get_locales(args: NominatimArgs, config: Configuration) -> napi.Locales:
     return napi.Locales()
 
 
-def _get_layers(args: NominatimArgs, default: napi.DataLayer) -> Optional[napi.DataLayer]:
+def _get_layers(args: NominatimArgs, default: Optional[napi.DataLayer]) -> Optional[napi.DataLayer]:
     """ Get the list of selected layers as a DataLayer enum.
     """
     if not args.layers:
@@ -193,7 +193,7 @@ class APISearch:
             raise UsageError(f"Unsupported format '{args.format}'. "
                              'Use --list-formats to see supported formats.')
 
-        layers = _get_layers(args, napi.DataLayer.ADDRESS | napi.DataLayer.POI)
+        layers = _get_layers(args, None)
 
         try:
             with napi.NominatimAPI(args.project_dir) as api:
@@ -262,7 +262,7 @@ class APIReverse:
         group.add_argument('--layer', metavar='LAYER',
                            choices=[n.name.lower() for n in napi.DataLayer if n.name],
                            action='append', required=False, dest='layers',
-                           help='OSM id to lookup in format <NRW><id> (may be repeated)')
+                           help='Restrict results to one or more layers (may be repeated)')
 
         _add_api_output_arguments(parser)
         _add_list_format(parser)