Try to address Gen-ART comments

1) Clarify the text in the 1st paragraph of Sever Behavior according to Gen-ART's comment
2) Add reference to RFC3046 in the section of Sever Behavior
3) DIDN'T follow the Gen-ART's comment to move RFC3046 & RFC3527 to Normative Reference, because this part "may" be implemented by vendors. Typically not required to implement.