[docs] LibraryEvolution: Fill out the section on fixed-contents classes.
[docs] LibraryEvolution: Fill out the section on fixed-contents classes.

Basically, elaborate on why it's not important right now. Also, drop the
section on limiting the addition of new vtable-dispatched methods. That's
not something we're really interested in.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift