Add more detail about metadata and the metaField (#5527)

* Add more detail about metadata and omitting this field.

* add newline

* external review changes

* rewording and moving to an include

Author: MongoCaleb

source/includes/time-series/fact-meta-field-description.rst

@@ -1,7 +1,14 @@
-Optional. The name of the field which contains metadata in each
-time series document. The metadata in the specified field should
-be data that is used to label a unique series of documents. The
-metadata should rarely, if ever, change.
+Optional. The name of the field which contains metadata in
+each time series document. The metadata in the specified field
+should be data that is used to label a unique series of
+documents. The metadata should rarely, if ever, change
+The name of the specified field may not be ``_id`` or the same
+as the ``timeseries.timeField``. The field can be of any type
+except array.
 
-The name of the specified field may not be ``_id`` or the same as
-the ``timeseries.timeField``. The field can be of any type.
+Although the ``metaField`` field is optional, using metadata can improve 
+query optimization. For example, MongoDB automatically 
+:ref:`creates a compound index <timeseries-add-secondary-index>` 
+on the ``metaField`` and ``timeField`` fields for new collections. 
+If you do not provide a value for this field, the data is bucketed solely 
+based on time. 

source/reference/command/create.txt

@@ -106,14 +106,7 @@ The ``create`` command has the following fields:
 
    * - ``timeseries.metaField``
      - string
-     - Optional. The name of the field which contains metadata in
-       each time series document. The metadata in the specified field
-       should be data that is used to label a unique series of
-       documents. The metadata should rarely, if ever, change.
- 
-       The name of the specified field may not be ``_id`` or the same
-       as the ``timeseries.timeField``. The field can be of any type
-       except array.
+     - .. include:: /includes/time-series/fact-meta-field-description.rst
 
    * - ``timeseries.granularity``