Clarify presentation ID generation requirements

[markafoltz]

Step 13 in starting a presentation (generation of presentation id) recommends a specific UUID algorithm. It would be better to clarify this as a "should" as it is a non-required part of an algorithm marked "must" instead of a generic "recommendation."

[tidoust]

See discussion at TPAC

ACTION: @mfoltzgoogle to check how other specs reference UUID generation and adjust the spec accordingly with SHOULD or MUST