time: delete incorrect docs about day-of-month checks.

time: delete incorrect docs about day-of-month checks.

Documentation was introduced by CL https://golang.org/cl/14123
but that behavior was changed later by CL https://golang.org/cl/17710.
This CL deletes the stale paragraph.

Fixes #18980

Change-Id: Ib434f1eac6fc814fde1be112a8f52afe6e3e0fcc
Reviewed-on: https://go-review.googlesource.com/36532
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>