2.11. Update Section 5.3.19.2 - Signing Key Pair Types

The following section clarifies the usage of the Signing Key Pair Types on referencing elliptic curves.

Insert this note at the end of Section 5.3.19.2 of [RFC4210]:

Note: In case several elliptic curves are supported, several id-ecPublicKey elements as defined in [RFC5480] need to be given, one per named curve.