unsafe: clarify when String bytes can be modified

They can be modified when the string returned by String no longer exists. Fixes #68300 Change-Id: I526a068151f7aad1ab6827504b51a84e7399cafe Reviewed-on: https://go-review.googlesource.com/c/go/+/596955 Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Than McIntosh <thanm@google.com> Commit-Queue: Ian Lance Taylor <iant@golang.org> Auto-Submit: Ian Lance Taylor <iant@golang.org> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ianlancetaylor

src/unsafe/unsafe.go

@@ -260,7 +260,7 @@ func SliceData(slice []ArbitraryType) *ArbitraryType
 // a run-time panic occurs.
 //
 // Since Go strings are immutable, the bytes passed to String
-// must not be modified afterwards.
+// must not be modified as long as the returned string value exists.
 func String(ptr *byte, len IntegerType) string
 
 // StringData returns a pointer to the underlying bytes of str.