Review number of MAY's in spec

[neilsjefferies]

Review all the MAY declarations in the specification to establish whether it might be tightened up (convert to MUSTs) for V2.0

[rosy1280]

we should create a list of the mays and review them one by one. this is something that we should do before we make any changes to the spec because it might identify something that we need to discuss for V2.