merge: pull request with additional information about $or operator

Author: Sam Kleinman

source/core/indexes.txt

@@ -796,3 +796,14 @@ Be aware of the following behaviors and limitations:
 
 - Create indexes to support queries and other operations, but do not
   maintain indexes that your MongoDB instance cannot or will not use.
+
+- For queries with the :operator:`$or` operator,
+  each clause of an :operator:`$or` query executes in parallel, and
+  can each use a different index.
+
+- For queries that use the :method:`sort() <cursor.sort()>` method and
+  use the :operator:`$or` operator, the query **cannot** use the
+  indexes on the :operator:`$or` fields.
+
+- ``2d`` :doc:`geospatial queries </core/geospatial-indexes>` do not
+  support queries that use the :operator:`$or` operator.