[docs] Add a note about where UBSan emits logs

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367270 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UndefinedBehaviorSanitizer.rst

@@ -224,6 +224,12 @@ will need to:
    ``UBSAN_OPTIONS=print_stacktrace=1``.
 #. Make sure ``llvm-symbolizer`` binary is in ``PATH``.
 
+Logging
+=======
+
+The default log file for diagnostics is "stderr". To log diagnostics to another
+file, you can set ``UBSAN_OPTIONS=log_path=...``.
+
 Silencing Unsigned Integer Overflow
 ===================================
 To silence reports from unsigned integer overflow, you can set