Document `AddNullMarkedToClass` and `AddNullMarkedToPackageInfo`

In 2.46.0 I get a lot of new warnings due to `AddNullMarkedToClass`. The link resolves to https://errorprone.info/bugpattern/AddNullMarkedToClass, which is empty. It would be helpful to get some instructions for this rule.

I marked all my classes using a package directive, so I have no idea what I am making wrong:

```
@DefaultAnnotation(NonNull.class)
package edu.hm.hafner.util;
```

See https://github.com/uhafner/codingstyle/runs/59898174755 and https://github.com/uhafner/codingstyle/pull/1714.