reflect: add missing String case in Value.UnsafePointer doc
CL 516596 changes Value.UnsafePointer to handle String case. However,
the method's doc comment is not updated to reflect this change.
Updates #61308
Change-Id: I84e02fd969ae0244184e1a2f05cac4651cdf7bff
Reviewed-on: https://go-review.googlesource.com/c/go/+/575956
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: Ian Lance Taylor <iant@google.com>