use simple numbers for return values in sub defer thread test
document clearer behavior for lazy attributes

When the attribute is lazy, it will be regenerated the next time it is
accessed. If it is not lazy, it will remain undef.
changelog for non-lazy clearer behavior docs
point to Type::Tiny rather than MooX::Types::MooseLike in docs
document Sub::Defer exports
note caveat on undefer_all