Add info for the field and restrict to 160 characters

- Could you rename the description of the field to Meta description to avoid confusion in the default setup
- Could you enclose default general guidelines for writing good meta descriptions; like: "A meta description should include as many keywords as possible, but still be a decent, readable text. The description should be no more than 160 characters long.
- Could you restrict the field to 160 characters?
- Maybe put in a counter, so one knows how many characters remains in the description