Reserving connection options

[mnot]

   When defining new connection options, specification authors ought to survey
   existing field names and ensure that the new connection option does
   not share the same name as an already deployed field.
   Defining a new connection option essentially reserves that potential
   field name for carrying additional information related to the
   connection option, since it would be unwise for senders to use
   that field name for anything else.

We should explicitly tell them to reserve it.