testing: document f.Fuzz requirement to not change underlying data

Updates #48606

Change-Id: I6d555fdefccd842fb65ec8d630b4808bcb54a825
Reviewed-on: https://go-review.googlesource.com/c/go/+/353977
Trust: Katie Hockman <[email protected]>
Run-TryBot: Katie Hockman <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>

src/testing/fuzz.go

@@ -299,11 +299,12 @@ var supportedTypes = map[reflect.Type]bool{
 // This function sould be fast and deterministic, and its behavior should not
 // depend on shared state. No mutatable input arguments, or pointers to them,
 // should be retained between executions of the fuzz function, as the memory
-// backing them may be mutated during a subsequent invocation.
+// backing them may be mutated during a subsequent invocation. ff must not
+// modify the underlying data of the arguments provided by the fuzzing engine.
 //
-// When fuzzing, F.Fuzz does not return until a problem is found, time runs
-// out (set with -fuzztime), or the test process is interrupted by a signal.
-// F.Fuzz should be called exactly once unless F.Skip or F.Fail is called.
+// When fuzzing, F.Fuzz does not return until a problem is found, time runs out
+// (set with -fuzztime), or the test process is interrupted by a signal. F.Fuzz
+// should be called exactly once, unless F.Skip or F.Fail is called beforehand.
 func (f *F) Fuzz(ff interface{}) {
  if f.fuzzCalled {
  panic("testing: F.Fuzz called more than once")