DOCSP-17077: Add time series index recommendation

Author: npentrel

source/core/timeseries/timeseries-secondary-index.txt

@@ -17,8 +17,13 @@ Add Secondary Indexes on metaField and timeField
    :keywords: Time series, IOT
 
 To improve query performance for :term:`time series collections <time
-series collection>`, add :term:`secondary indexes <secondary index>`
-that cover common query patterns.
+series collection>`, add one or more :term:`secondary indexes <secondary
+index>` to support common time series query patterns. Specifically, we
+recommend that you create one or more :ref:`compound indexes
+<index-type-compound>` on the fields specified as the ``timeField`` and
+the ``metaField``. If the field value for the ``metaField`` field is a
+document, you can create secondary indexes on fields inside that
+document.
 
 .. note::