pw_assert_log: fix pw_assert_HandleFailure to use FATAL

Fixes the pw_assert_HandleFailure implementation provided by
pw_assert_log by using PW_LOG_LEVEL_FATAL instead of
PW_LOG_LEVEL_CRITICAL.

Also updates the stale documentation to refer to the new
mechanism of using the fatal log level instead of a flag.

Change-Id: I738be4e5bea8307975001ab28b55999db32d6787
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/52480
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_assert_log/assert_log.cc b/pw_assert_log/assert_log.cc
index 278fa49..611f126 100644
--- a/pw_assert_log/assert_log.cc
+++ b/pw_assert_log/assert_log.cc
@@ -18,11 +18,11 @@
 
 extern "C" void pw_assert_HandleFailure(void) {
 #if PW_ASSERT_ENABLE_DEBUG
-  PW_LOG(PW_LOG_LEVEL_CRITICAL,
+  PW_LOG(PW_LOG_LEVEL_FATAL,
          PW_LOG_DEFAULT_FLAGS,
          "Crash: PW_ASSERT() or PW_DASSERT() failure");
 #else
-  PW_LOG(PW_LOG_LEVEL_CRITICAL,
+  PW_LOG(PW_LOG_LEVEL_FATAL,
          PW_LOG_DEFAULT_FLAGS,
          "Crash: PW_ASSERT() failure. Note: PW_DASSERT disabled");
 #endif  // PW_ASSERT_ENABLE_DEBUG