[docs] Improve UndefinedBehaviorSanitizer.rst

* Mention that -fsanitize= and -fno-sanitize= apply to check groups.
* Mention "all" can be used as a check group.
* Mention that -fsanitize-trap= and -fsanitize-recover= lead to no unused command line option warning.
* Mention that trap mode typically causes the program to terminate due to a `SIGILL` or `SIGTRAP` signal.

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D152650