document that events cannot be final

[marcpawl]

Classes that are wrapped by event cannot be final.

class A{}; class B final : public A {} ... make_transition_table( "before"_s + event<B> = "after"_s ); will not compile on gcc 8.2.1

[eadlam]

I just ran into this because I wanted to use protobuf messages as events, but can't because they are final.