Document special character handling in MONGO_*_URL

[hwillson]

Reference: meteor/meteor#7768

The Mongo Node driver can run into issues when using unencoded special characters in the MONGO_URL or MONGO_OPLOG_URL environment variables. We'll document this can happen, and recommend either not using special characters, or properly encoding the url.

[filipenevola]

I'm closing this issue because it's too old.

If you think this issue is still relevant please open a new one.