Documentation update

* certificate of type [**Certificate**](Ieee1609Dot2.md#Certificate)  OPTIONAL<br>

  contains the certificate that signed the PDUs if it is 

   not explicitly included in one of the PDUs. (There is no need to include 

   the entire certificate chain from the ITS station up to the Root CA, just 

   the ITS station certificate is enough, as the MA is expected to have the 

   rest of the certificates in the chain.) Note that if the sender certificate 

   changes, PDUs signed by the new certificate and included in this report 

   will be in a separate V2xPduStream instance within the v2xPduEvidence 

   field of the TemplateAsr.

   not explicitly included in one of the PDUs. For ETSI PDUs (i.e., PDUs with

   the psid field in the security headerInfo equal to an ITS-AID assigned to

   ETSI as per ETSI TS 102 965) the certificate shall be of type 

   EtsiTs103097Certificate as specified in ETSI TS 103 097. (There is no need

   to include the entire certificate chain from the ITS station up to the Root

   CA, just the ITS station certificate is enough, as the MA is expected to

   have the rest of the certificates in the chain.) Note that if the sender

   certificate changes, PDUs signed by the new certificate and included in

   this report will be in a separate V2xPduStream instance within the

   v2xPduEvidence field of the TemplateAsr.