encoding/base64: document that Strict mode still ignores newlines

An application that wants to reject non-canonical encodings is likely to
care about other sources of malleability.

Change-Id: I1d3a5b281d2631ca78df3f89b957a02687a534d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/188858


Reviewed-by: Katie Hockman <katie@golang.org>