Reintroduce HasLifecycleCallbacks annotation
Allow AlsoLoad annotation on methods without HasLifecycleCallbacks

AlsoLoad will be handled differently from actual lifecycle callback methods. See discussion in #428 for additional context.
Fix RST title syntax
Revise "Migrating Schemas" documentation
Document reintroduced HasLifecycleCallbacks documentation

This documentation was originally removed in abcf27564c3ed18f6fa6b8084d75e393cb30ca5c, after the annotation itself was removed in e9af24994055f94aede98593e7c2c421ee6a0a22.