test: add a comment on the effect of device DAXes order

Ref: #5615

Signed-off-by: Jan Michalski <jan.michalski@intel.com>

src/test/unittest/unittest.sh

@@ -1765,6 +1765,9 @@ function require_mmap_under_valgrind() {
  fatal "$FILE_MAX_DAX_DEVICES not found. Run make test."
  fi
 
+ # XXX The number of device DAXes available under this condition is dependent
+ # on the size and the order of device DAXes in the testconfig.sh file.
+ # https://github.com/pmem/pmdk/issues/5615
  if [ "$REQUIRE_DAX_DEVICES" -gt "$(< $FILE_MAX_DAX_DEVICES)" ]; then
  msg "$UNITTEST_NAME: SKIP: anonymous mmap under Valgrind not possible for $REQUIRE_DAX_DEVICES DAX device(s)."
  exit 0