Clarify template variable uniqueness

This clarifies that variables in both path and server URL templates
MUST be unique.  This is justified as compatible with our minor
release policy on the following grounds:

* Due to Parameter Object `name` + `in` constraints and the Server
  Object's `variables` field being a map, each variable can only
  map to one paramater or server variable (although the "one"
  paramater might be defined separately yet uniquely for each
  Operation).
* The Path Templating section uses the phrase "Each template
  expression in the path MUST correspond to ***a*** path parameter"
  (emphasis added).
* The Parameter Object uses similar language in the other direction:
  "If in is "path", the name field MUST correspond to ***a***
   template expression".
* The word "a" is interpreted to mean an unknown but unique thing
  in both directions.
* The Server Object's `url` template has always been assumed to
  function in an analogous way to path templating.