PHPLIB-663: Document aggregate "let" option

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

@@ -28,6 +28,12 @@ source:
 post: |
   .. versionadded:: 1.3
 ---
+source:
+  file: apiargs-aggregate-option.yaml
+  ref: let
+post: |
+  .. versionadded:: 1.9
+---
 source:
   file: apiargs-common-option.yaml
   ref: maxTimeMS

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

@@ -22,6 +22,12 @@ source:
   file: apiargs-common-option.yaml
   ref: hint
 ---
+source:
+  file: apiargs-aggregate-option.yaml
+  ref: let
+post: |
+  .. versionadded:: 1.9
+---
 source:
   file: apiargs-common-option.yaml
   ref: maxTimeMS

docs/includes/apiargs-aggregate-option.yaml

@@ -49,4 +49,18 @@ description: |
 interface: phpmethod
 operation: ~
 optional: true
+---
+arg_name: option
+name: let
+type: array|object
+description: |
+  Map of parameter names and values. Values must be constant or closed
+  expressions that do not reference document fields. Parameters can then be
+  accessed as variables in an aggregate expression context (e.g. ``$$var``).
+
+  This is not supported for server versions prior to 5.0 and will result in an
+  exception at execution time if used.
+interface: phpmethod
+operation: ~
+optional: true
 ...