updated doc for ViewMode == NoDisplay

docs/settings.md

@@ -178,6 +178,7 @@ The ViewMode determines how you will view the vehicle. For multirotors, the defa
 * Fpv: View the scene from front camera of vehicle
 * Manual: Don't move camera automatically. Use arrow keys and ASWD keys for move camera manually.
 * SpringArmChase: Chase the vehicle with camera mounted on (invisible) arm that is attached to the vehicle via spring (so it has some latency in movement).
+* NoDisplay: This will freeze rendering for main screen however rendering for subwindows, recording and APIs remain active. This mode is useful to save resources in "headless" mode where you are only interested in getting images and don't care about what gets rendered on main screen. This may also improve FPS for recording images.
 
 ### TimeOfDay
 This section of settings controls the position of Sun in the environment. By default `Enabled` is false which means Sun's position is left at whatever was the default in the environment and it doesn't change over the time. If `Enabled` is true then Sun position is computed using longitude, latitude and altitude specified in `OriginGeopoint` section for the date specified in `StartDateTime` in the string format as `%Y-%m-%d %H:%M:%S`, for example, `2018-02-12 15:20:00`. If this string is empty then current date and time is used. If `StartDateTimeDst` is true then we adjust for day light savings time. The Sun's position is then contibuously updated at the interval specified in `UpdateIntervalSecs`. In some cases, it might be desirable to have celestial clock run faster or slower than simulation clock. This can be specified using `CelestialClockSpeed`, for example, value 100 means for every 1 second of simulation clock, Sun's position is advanced by 100 seconds so Sun will move in sky much faster.