kcsan: Update Documentation/dev-tools/kcsan.rst

Extend and improve based on recent changes, and summarize important
bits that have been missing. Tested with "make htmldocs".
Signed-off-by: Marco Elver <elver@google.com>
Cc: Qian Cai <cai@lca.pw>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>