Clarify when aggregate and mapReduce options may be ignored

docs/includes/apiargs-MongoDBCollection-method-aggregate-option.yaml

@@ -70,6 +70,8 @@ source:
 source:
   file: apiargs-MongoDBCollection-common-option.yaml
   ref: readPreference
+post: |
+  This option will be ignored when using the :ref:`$out <agg-out>` stage.
 ---
 source:
   file: apiargs-MongoDBCollection-common-option.yaml

docs/includes/apiargs-MongoDBCollection-method-mapReduce-option.yaml

 source:
   file: apiargs-MongoDBCollection-common-option.yaml
   ref: bypassDocumentValidation
+post: |
+  This only applies when results are output to a collection.
 ---
 source:
   file: apiargs-MongoDBCollection-common-option.yaml
@@ -55,6 +57,8 @@ source:
 source:
   file: apiargs-MongoDBCollection-common-option.yaml
   ref: readPreference
+post: |
+  This option will be ignored when results are output to a collection.
 ---
 arg_name: option
 name: scope

src/Operation/Aggregate.php

@@ -92,6 +92,8 @@ class Aggregate implements Executable
      *
      *  * readPreference (MongoDB\Driver\ReadPreference): Read preference.
      *
+     *    This option is ignored if the $out stage is specified.
+     *
      *  * typeMap (array): Type map for BSON deserialization. This will be
      *    applied to the returned Cursor (it is not sent to the server).
      *

src/Operation/MapReduce.php

@@ -107,6 +107,8 @@ class MapReduce implements Executable
      *
      *  * readPreference (MongoDB\Driver\ReadPreference): Read preference.
      *
+     *    This option is ignored if results are output to a collection.
+     *
      *  * scope (document): Specifies global variables that are accessible in
      *    the map, reduce and finalize functions.
      *