[flang][test-suite] Document need for NO_STOP_MESSAGE environment var…

…iable. NFC

When running the llvm-test-suite with flang, we get a lot of failures
because of the output of the `STOP` statement. We can workaround them by
setting `NO_STOP_MESSAGE=1` in the environment. This patch adds a few
words about it to the docs about the Fortran part of the llvm-test-suite.

See also https://reviews.llvm.org/D126360

flang/docs/FortranLLVMTestSuite.md

@@ -34,6 +34,10 @@ are found in the Fortran subdirectory. To run the C/C++ tests
 alongside the Fortran tests omit the `-DTEST_SUITE_SUBDIRS` CMake
 variable.
 
+If your Fortran compiler is Flang, you may want to set the `NO_STOP_MESSAGE`
+environment variable to `1` in order to avoid test failures due to warnings
+about INEXACT signaling exceptions.
+
 
 ## Running the SPEC CPU 2017