cmd/cgo: clarify C.GoStringN and C.GoBytes docs

Fixes #12427.

Change-Id: I46725620c1e163f3b60ffcd85e5388fa646f074d
Reviewed-on: https://go-review.googlesource.com/15997Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cgo/doc.go

@@ -189,10 +189,10 @@ by making copies of the data.  In pseudo-Go definitions:
 	// C string to Go string
 	func C.GoString(*C.char) string
 
-	// C string, length to Go string
+	// C data with explicit length to Go string
 	func C.GoStringN(*C.char, C.int) string
 
-	// C pointer, length to Go []byte
+	// C data with explicit length to Go []byte
 	func C.GoBytes(unsafe.Pointer, C.int) []byte
 
 C references to Go