PHPLIB-577: Add examples for aggregation methods (#797)

* Fix wrong collection name in aggregation example

* Add example for MongoDB\Collection::aggregate

* Add example for MongoDB\Database::aggregate

Author: alcaeus

docs/reference/method/MongoDBCollection-aggregate.txt

@@ -58,7 +58,25 @@ MongoDB server version and whether the ``useCursor`` option is specified. If
 ``result`` array from the command response document. In both cases, the return
 value will be :php:`Traversable <traversable>`.
 
-.. todo: add examples
+Examples
+--------
+
+The following aggregation example uses a collection called ``names`` and groups
+the ``first_name`` field together, counts the total number of results in each
+group, and sorts the results by name.
+
+.. code-block:: php
+
+   <?php
+
+   $collection = (new MongoDB\Client)->test->names;
+
+   $cursor = $collection->aggregate(
+       [
+           ['$group' => ['_id' => '$first_name', 'name_count' => ['$sum' => 1]]],
+           ['$sort' => ['_id' => 1]],
+       ]
+   );
 
 See Also
 --------

docs/reference/method/MongoDBDatabase-aggregate.txt

@@ -51,7 +51,25 @@ Errors/Exceptions
 
 .. _php-db-agg-method-behavior:
 
-.. todo: add examples
+Examples
+--------
+
+The following aggregation example lists all running commands using the
+``$currentOp`` aggregation pipeline stage, then filters this list to only show
+running command operations.
+
+.. code-block:: php
+
+   <?php
+
+   $database = (new MongoDB\Client)->admin;
+
+   $cursor = $database->aggregate(
+       [
+           ['$currentOp' => []],
+           ['$match' => ['op' => 'command'],
+       ]
+   );
 
 See Also
 --------

docs/tutorial/collation.txt

@@ -360,7 +360,7 @@ group, and sorts the results by German phonebook order.
 
    <?php
 
-   $collection = (new MongoDB\Client)->test->recipes;
+   $collection = (new MongoDB\Client)->test->names;
 
    $cursor = $collection->aggregate(
        [