Add package info to javadoc for new packages in spring-boot-loader(-tools)

[wilkinsona]

The following packages have no description:

• org.springframework.boot.loader.jarmode
• org.springframework.boot.loader.tools.layer
• org.springframework.boot.loader.tools.layer.classes
• org.springframework.boot.loader.tools.layer.library

[cicioflaviu]

Can I work on this one?

[wilkinsona]

Thanks for the offer, @cicioflaviu. We have some renaming planned for at least some of those packages (#20526) so I think it's probably better if we take care of this ourselves as part of sorting that out.