Expand AutowireMethodOf description

[javiereguiluz]

In #19819 we added the description of the new #[AutowireMethodOf] attribute.

The new content is correct, but I think it's missing useful tips and contents included in the symfony/symfony#54016 discussion:

• When/why use this instead of injecting the dependency directly? ([DependencyInjection] Add #[AutowireMethodOf] attribute to autowire a method of a service as a callable symfony#54016 (comment))
• This supposedly simplifies tests a lot ([DependencyInjection] Add #[AutowireMethodOf] attribute to autowire a method of a service as a callable symfony#54016 (comment)) but I'd like to see a real test example because I don't fully understand this.
• Maybe also include this factory example? [DependencyInjection] Add #[AutowireMethodOf] attribute to autowire a method of a service as a callable symfony#54016 (comment)