add documentation for new query preprocessing

src/nominatim_api/query_preprocessing/normalize.py

@@ -5,7 +5,12 @@
 # Copyright (C) 2024 by the Nominatim developer community.
 # For a full list of authors see the git log.
 """
-Normalize query test using an ICU transliterator.
+Normalize query text using the same ICU normalization rules that are
+applied during import. If a phrase becomes empty because the normalization
+removes all terms, then the phrase is deleted.
+
+This preprocessor does not come with any extra information. Instead it will
+use the configuration from the `normalization` section.
 """
 from typing import cast