pathpol: Update design doc (#3070)
pathpol: Update design doc (#3070)

Mention how policy filtering in PS works.
Also mention relation between acl and sequence.

Fixes #3063
netsec-ethz/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion