[UWS] Fix missing xlink:type (though it is optional) in jobRef (in XML job list).

This commit also includes the 2 following points:

    - for (really really tiny) optimisation purpose, replace String by a
	  char when there is only one character to append to a StringBuffer

	- add a bit more details about Result.type in the Javadoc (it should be
	  a xlink:type and not a random custom type)