Document that assertThrows allows exception subtypes

[Marcono1234]

Overview

Currently the assertThrows documentation does not mention that exception subtypes will cause the assertion to succeed as well. This caused some confusion for #2505 (and lead to the introduction of assertThrowsExactly).

---

I hereby agree to the terms of the JUnit Contributor License Agreement.

---

Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[marcphilipp]

@Marcono1234 Thank you for your contribution!