The docs, help text, and code for --writeConcern are inconsistent.

• Docs: say it takes a "<document>", but says defaults to 'majority`, rather than {"w": "majority"}. Says it supports "w options" which implies only "w" and not other write concern options.
• The command line help text says it takes "<write concern"> and gives two examples:

  --writeConcern majority
  --writeConcern '{w: 3, wtimeout: 500, fsync: true, j: true}'
  

• The Go driver doesn't support fsync, nor is it documented in server docs. Supposedly, it works the same as "j" for servers with journaling (see DRIVERS-125).

We should decide exactly which keys we support (now excluding 'fsync') and ensure that code, docs and CLI help text all agree.