doc/go1.17: document time changes for Go1.17

Documents the newly implemented changes of
- Time.IsDST() method
- Addition of Time.UnixMilli, Time.UnixMicro and to-Time helpers UnixMicro, UnixMilli methods
- Addition of comma "," support as separator for fraction seconds

For #44513
Fixes #46026

Change-Id: Ib8d3449d3b061f013112d33362b50e68ad6ddffa
Reviewed-on: https://go-review.googlesource.com/c/go/+/317913


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>