DOCSP-41173 misc docs updates (#674)

Author: kyuan-mongodb

source/dashboards/view-dashboard-data/dashboard-filtering.txt

@@ -57,6 +57,18 @@ filtered fields impacting that chart.
    - A dashboard ``Author`` or ``Owner`` :ref:`disables filtering
      <disable-filtering-on-chart>` on some charts.
 
+Considerations
+~~~~~~~~~~~~~~
+
+- Dashboard filters don't support :ref:`lookup fields
+  <add-lookup-field>`, :ref:`calculated fields <calculated-fields>` and
+  :ref:`missing fields <missing-fields>`. If you want to add a lookup or
+  calculated field, create a :ref:`charts view <charts-views>` and use it
+  as a :ref:`data source <data-sources>`. 
+
+- If you have filters on a chart or dashboard, use an :manual:`index
+  </indexes/>` to improve its performance.
+
 Example Scenario
 ~~~~~~~~~~~~~~~~
 
@@ -344,6 +356,9 @@ To filter dashboard data based on filterable fields:
       If the filter card field is a ``String`` or ``ObjectId``, 
       |charts-short| only samples field values from the first data 
       source that was added to the filter. 
+   
+   The dashboard filter shows 20 sample values. If you want to add your
+   own value, click :guilabel:`+ Add Value` and enter the value.
 
    Updating the filter value refines chart data based on all data 
    sources linked to the filter card.

source/manage-data-sources.txt

@@ -24,6 +24,10 @@ On the :guilabel:`Data Sources` page, an |service|
   |charts|.
 - Modify :ref:`data source settings <access-data-source-settings-page>`.
 
+.. note:: 
+
+   If you want to combine two or more data sources, you must create an {+adf+}.
+
 .. _add-data-source:
 .. _connect-data-source: