Update the spec to reflect the var -> ref change

What it says on the tin. I believe I've reworded all the references to var functions that are in the spec.
While here, let's fix "defined in the class with the same name as the field", and a couple of other things. E.g.:

All field accesses are performed via getters.
A getter is a method without parentheses with the same name as the
field. It is defined in the field's class and has
a \chpl{ref} return intent (\rsec{Ref_Return_Intent}).
If the program does not define it, the default getter,
which simply returns the field, is provided.