doc/go1.16: reference misc/ios/README for how to build iOS programs

Updates #43371, #43343.

Change-Id: Ib89b809a5220717507272453ea86224d1928dd36
Reviewed-on: https://go-review.googlesource.com/c/go/+/280154


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>