SITL Github Action: Document Grind config in-place

This helps anyone needing it to find and set the correct grind iteration parameter.

.github/workflows/sitl_tests.yml

@@ -19,5 +19,5 @@ jobs:
       # Build all targets
       run: DONT_RUN=1 make px4_sitl gazebo mavsdk_tests
     - name: Run simulation tests
-      # Grind configuration: Run ~8 minutes for 35 iterations == close to the 6 hour limit of Github Actions
+      # Grind configuration: Change iterations to a number between 20 and 100 (30 minutes to about 3 hours runtime)
       run:  test/mavsdk_tests/mavsdk_test_runner.py --speed-factor 20 --iterations 1 --fail-early