Document what does it mean for ServletFilterHolder.getDispatcherType() to return null

`ServletFilterHolder.getDispatcherType()` is annotated `@Nullable` but it doesn't explain in what cases it's OK (or not OK) to return null from this method for subclasses.

Find the ServletFilterHolder class and inspect getDispatcherType(), its @Nullable annotation, and subclass implementations. Document when returning null is valid or invalid for subclasses, then verify the method documentation clearly explains the expected behavior.

Written by the indexing model from the issue text.