spec: document the new unsafe functions SliceData, String, and StringData
For #53003.
Change-Id: If5d76c7b8dfcbcab919cad9c333c0225fc155859
Reviewed-on: https://go-review.googlesource.com/c/go/+/449537 Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@google.com>