Revise change stream doc examples to use rewind()
Revise change stream doc examples to use rewind()

This also adds more assertions between the doc examples.

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