spec: permit trailing comma in conversions
spec: permit trailing comma in conversions

Also: Adjust prose in Conversions section
to be consistent.

This is a backward-compatible syntax change.
For a discussion see issue 2568.

Fixes issue 2568.
Fixes issue 4123.

R=golang-dev, r, iant, rsc
CC=golang-dev
http://codereview.appspot.com/6572049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang