ci: Document the false positive error for g++-9

Author: MarcoFalke

ci/test/00_setup_env_native_qt5.sh

@@ -15,7 +15,7 @@ export TEST_RUNNER_EXTRA="--previous-releases --coverage --extended --exclude fe
 export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"
 export GOAL="install"
-export NO_WERROR=1
+export NO_WERROR=1  # -Werror=maybe-uninitialized
 export DOWNLOAD_PREVIOUS_RELEASES="true"
 export BITCOIN_CONFIG="--enable-zmq --with-libs=no --with-gui=qt5 --enable-reduce-exports \
 --enable-debug CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\""