Improve AlsoLoad annotation documentation

Discuss how AlsoLoad may be used for methods, and link to migrations page.
Handle AlsoLoad methods like properties and without HasLifecycleCallbacks

AlsoLoad method annotations will no longer require the HasLifecycleCallbacks class annotation. The latter will just be used for lifecycle callbacks.

To be consistent with property handling, an AlsoLoad method may be registered with multiple fields. Fields will be considered in order and the method will be invoked at most once with the first value found.

Wrote additional functional tests for AlsoLoad behavior for both properties and methods.
Inherit AlsoLoad methods from parent's ClassMetadata