add details about what executionTimeMillis includes (#5863)

* add details about what executionTimeMillis includes

* review suggestions

* Address review suggestions

* remove antiquated info

Author: MongoCaleb

source/reference/explain-results.txt

@@ -12,8 +12,8 @@ Explain Results
    :depth: 2
    :class: singlecol
 
-To return information on query plans and execution statistics of the
-query plans, MongoDB provides:
+To return information on :ref:`query plans <query-plans-query-optimization>` and 
+execution statistics of the query plans, MongoDB provides:
 
 - the :method:`db.collection.explain()` method,
 
@@ -565,9 +565,17 @@ representative. Your output may differ significantly.
    .. data:: explain.executionStats.executionTimeMillis
 
       Total time in milliseconds required for query plan selection and
-      query execution. :data:`explain.executionStats.executionTimeMillis` corresponds
-      to the ``millis`` field returned by ``cursor.explain()`` in
-      earlier versions of MongoDB.
+      query execution. It includes the time it takes to run the trial phase 
+      part of the plan selection process, but does not include the network time 
+      to transmit the data back to the client.
+
+      The time reported by ``explain.executionStats.executionTimeMillis`` is 
+      not necessarily representative of actual query time. During steady 
+      state operations (when the query plan is cached), or when using 
+      :method:`cursor.hint()` with ``cursor.explain()``, MongoDB bypasses the 
+      plan selection process, resulting in a faster actual time, leading to 
+      a lower ``explain.executionStats.executionTimeMillis`` value.
+
 
    .. data:: explain.executionStats.totalKeysExamined