pw_cpu_exception_cortex_m: Add deeper exception information

* Add more informational text about exception flags from the ARM user
guides
* Print the informational text upon exception analysis
* Update documentation example
* Update cortex-m exception analyzer unit test

Change-Id: I1fe27216681ad4eb02b25eaf29d231dad7484e83
Signed-off-by: Shiva Rajagopal <shivarajagopal@google.com>
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44565
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>