reflect: enhance docs for IsZero and IsValid

Make it clear that IsValid checks that we have valid
reflect.Value and not the value of `v`

fixes #34152

Change-Id: Ib3d359eeb3a82bf733b9ed17c777fc4c143bc29c
Reviewed-on: https://go-review.googlesource.com/c/go/+/193841Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/reflectlite/value.go

@@ -305,7 +305,7 @@ func (v Value) IsNil() bool {
 // IsValid reports whether v represents a value.
 // It returns false if v is the zero Value.
 // If IsValid returns false, all other methods except String panic.
-// Most functions and methods never return an invalid value.
+// Most functions and methods never return an invalid Value.
 // If one does, its documentation states the conditions explicitly.
 func (v Value) IsValid() bool {
 	return v.flag != 0

src/reflect/value.go

@@ -1076,7 +1076,7 @@ func (v Value) IsNil() bool {
 // IsValid reports whether v represents a value.
 // It returns false if v is the zero Value.
 // If IsValid returns false, all other methods except String panic.
-// Most functions and methods never return an invalid value.
+// Most functions and methods never return an invalid Value.
 // If one does, its documentation states the conditions explicitly.
 func (v Value) IsValid() bool {
 	return v.flag != 0