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>