DOCSP-15298 doc search as subpipeline of lookup and unionWith (#1177)

* DOCSP-15298 doc search as subpipeline of lookup and unionWith

* DOCSP-15298 updates for CR feedback

* DOCSP-15298 updates to increase depth of toc

Author: kanchana-mongodb

source/reference/operator/aggregation/lookup.txt

@@ -7,7 +7,7 @@ $lookup (aggregation)
 .. contents:: On this page
    :local:
    :backlinks: none
-   :depth: 1
+   :depth: 2
    :class: singlecol
 
 Definition
@@ -206,7 +206,10 @@ The :pipeline:`$lookup` stage accepts a document with these fields:
        ``pipeline`` ``[]``.
 
        The ``pipeline`` cannot include the :pipeline:`$out` stage or the
-       :pipeline:`$merge` stage.
+       :pipeline:`$merge` stage. Starting in v6.0, the ``pipeline`` 
+       can contain the :atlas:`Atlas Search </atlas-search>` 
+       :pipeline:`$search` stage as the first stage inside the 
+       pipeline. To learn more, see :ref:`fts-lookup-behavior`.
 
        The ``pipeline`` cannot directly access the joined document
        fields. Instead, define variables for the joined document fields
@@ -332,7 +335,10 @@ The :pipeline:`$lookup` accepts a document with these fields:
        return all documents, specify an empty ``pipeline`` ``[]``.
 
        The ``pipeline`` cannot include the :pipeline:`$out` or
-       :pipeline:`$merge` stages.
+       :pipeline:`$merge` stages. Starting in v6.0, the ``pipeline`` 
+       can contain the :atlas:`Atlas Search </atlas-search>` 
+       :pipeline:`$search` stage as the first stage inside the 
+       pipeline. To learn more, see :ref:`fts-lookup-behavior`.
 
        The ``pipeline`` cannot directly access the document fields.
        Instead, define variables for the document fields using the
@@ -399,6 +405,39 @@ the ``pipeline`` field.
       }
    }
 
+.. _fts-lookup-behavior:
+
+Atlas Search Support 
+~~~~~~~~~~~~~~~~~~~~
+
+Starting in v6.0, the ``pipeline`` can contain the :atlas:`Atlas Search 
+</atlas-search>` :pipeline:`$search` stage as the first stage inside 
+the pipeline. 
+
+For example, when you :ref:`lookup-syntax-let-pipeline` or run 
+:ref:`lookup-syntax-concise-correlated-subquery`, you can specify 
+:pipeline:`$search` inside the pipeline as shown below:
+
+.. code-block:: 
+   :copyable: false 
+   :emphasize-lines: 7-11
+
+   [{
+     "$lookup": {
+       "from": <joined collection>,
+       "let": {<var_n>: <expression>, ...},
+       "as": <output array field>,
+       "pipeline": [{
+         "$search": {
+           "<operator>": {
+             <operator-specification>
+           }
+         },
+         ... 
+       }]
+     }
+   }] 
+
 .. _lookup-sharded-collections:
 
 Sharded Collections

source/reference/operator/aggregation/unionWith.txt

@@ -7,7 +7,7 @@ $unionWith (aggregation)
 .. contents:: On this page
    :local:
    :backlinks: none
-   :depth: 1
+   :depth: 2
    :class: singlecol
 
 Definition
@@ -69,7 +69,10 @@ fields:
       ``[ <stage1>, <stage2>, ...]``
 
       The pipeline cannot include the :pipeline:`$out` and
-      :pipeline:`$merge` stages.
+      :pipeline:`$merge` stages. Starting in v6.0, the ``pipeline`` 
+      can contain the :atlas:`Atlas Search </atlas-search>` 
+      :pipeline:`$search` stage as the first stage inside the pipeline. 
+      To learn more, see :ref:`fts-unionwith-behavior`.
 
 The :pipeline:`$unionWith` operation would correspond to the following
 SQL statement:
@@ -204,6 +207,33 @@ top-level collection/view on which the
   </core/views>`, then its collation must match that of the top-level
   collection/view.  Otherwise, the operation errors.
 
+.. _fts-unionwith-behavior:
+
+Atlas Search Support
+~~~~~~~~~~~~~~~~~~~~
+
+Starting in v6.0, the ``pipeline`` can contain the :atlas:`Atlas Search 
+</atlas-search>` :pipeline:`$search` stage as the first stage inside 
+the pipeline.
+
+.. code-block:: 
+   :copyable: false 
+   :emphasize-lines: 5-9
+
+   [{
+     "$lookup": {
+       "coll": <collection-name>,
+       "pipeline": [{
+         "$search": {
+           "<operator>": {
+             <operator-specification>
+           }
+         },
+         ... 
+       }]
+     }
+   }] 
+
 Restrictions
 ~~~~~~~~~~~~