[docs][Fortran] Document FCVS_ALLOW_FLEXIBLE_OUTPUT

[rovka]

Summary

Add a short explanation of the FCVS_ALLOW_FLEXIBLE_OUTPUT cmake option
for the llvm-test-suite.

See also https://reviews.llvm.org/D131754 and https://reviews.llvm.org/D128261

flang/docs/FortranLLVMTestSuite.md

 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.
 
+Note that the reference outputs are by default matched exactly. If you see
+failures in FCVS when running the test-suite, you might want to try configuring
+it with `-DFCVS_ALLOW_FLEXIBLE_OUTPUT:BOOL=ON`. This will relax the checking for
+some of the tests which are known to be problematic (usually because they use
+Fortran features that are not very strictly specified in the standard, such as
+list-directed output).
+
 
 ## Running the SPEC CPU 2017
 
 We recently added CMake hooks into the LLVM Test Suite to support
 Fortran tests from [SPEC CPU 2017](https://www.spec.org/cpu2017/). We
 strongly encourage the use of the CMake Ninja (1.10 or later) generator
 due to better support for Fortran module dependency detection. Some of
 the SPEC CPU 2017 Fortran tests, those that are derived from climate