We should clarify that the default for --fork is false, and that reasonable cases exist for turning it off other than testing, such as the calling process handling daemonization. For example upstart by default, start-stop-daemon in Debian with --background, and the homebrew default wrapper.

http://docs.mongodb.org/master/reference/program/mongod/#cmdoption--fork

Current text is:
"Enables a daemon mode for mongod that runs the process in the background. This is the normal mode of operation in production and production-like environments but may not be desirable for testing."