reflect: document ptrdata field of rtype

This CL adds a simple explanation about what means the ptrdata field of
the reflect.rtype type.
Also document that rtype needs to be kept in sync with the runtime._type
type that rtype mirrors.

Change-Id: Icd9663a2e4bb94d922a2417cfe4537861d2ccc97
Reviewed-on: https://go-review.googlesource.com/40917Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/type.go

@@ -289,9 +289,11 @@ const (
 // It is embedded in other, public struct types, but always
 // with a unique tag like `reflect:"array"` or `reflect:"ptr"`
 // so that code cannot convert from, say, *arrayType to *ptrType.
+//
+// rtype must be kept in sync with ../runtime/type.go:/^type._type.
 type rtype struct {
 	size       uintptr
-	ptrdata    uintptr
+	ptrdata    uintptr  // number of bytes in the type that can contain pointers
 	hash       uint32   // hash of type; avoids computation in hash tables
 	tflag      tflag    // extra type information flags
 	align      uint8    // alignment of variable with this type