PHPLIB-438: Update aggregation docblocks relating to write stages
PHPLIB-438: Update aggregation docblocks relating to write stages

10gen-labs/mongo-php-library-prototype,jmikola/mongo-php-library,jmikola/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library,mongodb/mongo-php-library,jmikola/mongo-php-library-prototype,mongodb/mongo-php-library