Cover the short form match arm syntax Point { x, y } in addition to Point { x: x, y: y }

The patterns.md page doesn't explain that it's possible to match compound data in this way:
 `match origin { Point { x, y, .. } => ()`
Currently only this form is covered:
`match origin { Point { x: x, y: y, .. } => ()`

The latter form results in compiler warnings (`warning: the y: in this pattern is redundant and can be removed`). I'm not sure whether it should be avoided for that reason.
