Revise MongoCollection ensure/createIndex docs

Documented options for MongoDB 2.6 and earlier versions and clarified how methods operate with each database version. Also updated write concern snippets for consistent language.

https://jira.mongodb.org/browse/PHP-1078


git-svn-id: https://svn.php.net/repository/phpdoc/doc-base/trunk@333382 c90b9560-bf6c-de11-be94-00142212c4b1