User Guide: Type-use annotation syntax for full qualified names

[bjmi]

This is a follow up of https://groups.google.com/g/jspecify-discuss/c/eCiRwkUC52k/m/K8YHKkUcAgAJ

https://jspecify.dev/docs/user-guide#type-use-annotation-syntax mentions type use syntax for nested static type and array type.

Sometimes it is necessary to write the full qualified name of a type e.g. because of name clash thus the syntax is
java.lang.@Nullable String instead of @Nullable java.lang.String

A full qualified name example could help jspecify adopters that aren't familiar with type use annotation syntax.

Hence, please add a short example for this use case.

[kevinb9n]

In fact another easy way this can happen is if using Map.Entry and Multiset.Entry in the same file.

In general, we may have documented a number of things as though the reader knows all about annotations already. But the more JSpecify were to be embraced the more that it might be how a user is introduced to annotations.