docs: Update property requirement specifications (#470)

* docs: Update property requirement specifications

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google/cloud/datastore_v1/types/query.py

@@ -455,9 +455,13 @@ class PropertyReference(proto.Message):
 
     Attributes:
         name (str):
-            The name of the property.
-            If name includes "."s, it may be interpreted as
-            a property name path.
+            A reference to a property.
+
+            Requires:
+
+            -  MUST be a dot-delimited (``.``) string of segments, where
+               each segment conforms to [entity property
+               name][google.datastore.v1.Entity.properties] limitations.
     """
 
     name: str = proto.Field(