DDSXTY13 — Clarify which of the options bits are set to indicate padding bytes

As the spec is currently written, section 7.6.2.1.2
'Use of the RTPS Encapsulation Identifier', it is ambiguous as to which bits are used to encode the padding bytes.

RTPS 2.4 is redefining the options field to be an array of two octets rather than a ushort. So XTypes should specify that the lower-order two bits of options[1] are used to encode the number of padding bytes.

Modify 7.6.2.1.2 'Use of the RTPS Encapsulation Identifier'

Modify 7.6.2.1.2 'Use of the RTPS Encapsulation Identifier' to indicate that the options field is octet[2] and that the bits using for padding are the least significant bits of the second byte:

0...2...........8...............16..............24..............32 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
|   representation_identifier   |               |            X X| 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+