perlunicode: Note Bidi_Class changed in Unicode 6.3

perlunicode: Note Bidi_Class changed in Unicode 6.3

This property had been stable for many Unicode releases, but it is not
guaranteed to be stable, and in fact changed in 6.3.  I left in the
existing list of property values (noting that others have been added)
because they are referred to elsewhere in the pod, and provide a
reasonable example of how multi-valued properties work.

The text used the term "property" to refer to what is properly called
"property values", and used the term "class" to refer to what should
have been called "property".  There is another instance of this
confusion of terms in the pod, which will be corrected in a future
commit