clarify the change in flow for NA(EARO) for RULs

[nyrahul]

Storing-Root-Ack needs to handle the case where RULs are trying to attach to the 6LR.
The RUL should be informed of the NA only once the end to end RPL path is established. Thus the NA to the RUL should be sent on receiving the storing-root-ack.

Quoting Pascal's comment here for ref:
"The RUL is not aware of RPL so it cannot know whether the 6LR is doing storing-root-ack or even talking RPL at all for that matter.
What makes sense is that on the first registration, the 6LR (optionally) waits for the storing-root-ack before it sends the NA(EARO) back. This change of the flow would be indicated in your storing-root-ack draft."

[rabinsahoo]

If 6LR wait for the storing-root-ack, before it can send NA to confirm the address registration, it has to handle the negative storing-root-ack too. A new status code need to be added in (EARO)as part of this change.