docs: Add undefined to --with-sanitizers=fuzzer,address

Author: practicalswift

doc/fuzzing.md

@@ -77,13 +77,13 @@ will print an error and suggestion if so.
 
 ## libFuzzer
 
-A recent version of `clang`, the address sanitizer and libFuzzer is needed (all
+A recent version of `clang`, the address/undefined sanitizers (ASan/UBSan) and libFuzzer is needed (all
 found in the `compiler-rt` runtime libraries package).
 
 To build all fuzz targets with libFuzzer, run
 
 ```
-./configure --disable-ccache --enable-fuzz --with-sanitizers=fuzzer,address CC=clang CXX=clang++
+./configure --disable-ccache --enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=clang CXX=clang++
 make
 ```