clarify when header field combination is allowed

[mnot]

Fixes #74

[royfielding]

I had in mind something a little more general, that would include fields which are defined like Vary:

   Aside from the well-known exception noted below,
   a sender MUST NOT generate multiple header fields with the same field
   name in a message, or append a header field when a field of the same name
   already exists in the message, unless that field's definition allows multiple
   field values to be recombined as a comma-separated list [i.e., at least one
   alternative of the field's definition allows a comma-separated list, such as
   an ABNF rule of #(values)].