kcsan: Clarify Kconfig option KCSAN_IGNORE_ATOMICS

Clarify difference between options KCSAN_IGNORE_ATOMICS and
KCSAN_ASSUME_PLAIN_WRITES_ATOMIC in help text.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

lib/Kconfig.kcsan

@@ -132,8 +132,18 @@ config KCSAN_ASSUME_PLAIN_WRITES_ATOMIC
 config KCSAN_IGNORE_ATOMICS
 	bool "Do not instrument marked atomic accesses"
 	help
-	  If enabled, never instruments marked atomic accesses. This results in
-	  not reporting data races where one access is atomic and the other is
-	  a plain access.
+	  Never instrument marked atomic accesses. This option can be used for
+	  additional filtering. Conflicting marked atomic reads and plain
+	  writes will never be reported as a data race, however, will cause
+	  plain reads and marked writes to result in "unknown origin" reports.
+	  If combined with CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=n, data
+	  races where at least one access is marked atomic will never be
+	  reported.
+
+	  Similar to KCSAN_ASSUME_PLAIN_WRITES_ATOMIC, but including unaligned
+	  accesses, conflicting marked atomic reads and plain writes will not
+	  be reported as data races; however, unlike that option, data races
+	  due to two conflicting plain writes will be reported (aligned and
+	  unaligned, if CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n).
 
 endif # KCSAN