Invalid addresses might not match. (#911)

* Clarify matching on invalid virtual addresses behavior

See:
* https://lists.riscv.org/g/tech-debug/message/1407
* https://lists.riscv.org/g/tech-debug/files/meetings%202023/2023-01-10%20Debug%20Task%20Group%20Call%20Notes.pdf
* https://lists.riscv.org/g/tech-debug/files/meetings%202023/2023-01-10%20Debug%20Task%20Group%20Call%20Slides.pdf

* Allow invalid addresses not to match at all.

Also clarify tdata2 requirements for triggers that are only used for
address matches.