Merge pull request #463 from defold/headless-update-frequency

Added note about display.update_frequency for headles builds

docs/en/manuals/project-settings.md

@@ -108,6 +108,7 @@ Check if the application should start full screen. If unchecked, the application
 
 #### Update Frequency
 The desired frame rate in Hertz. Set to 0 for variable frame rate. A value larger than 0 will result in a fixed frame rate capped at runtime towards the actual frame rate (which means that you cannot update the game loop twice in an engine frame). Use [`sys.set_update_frequency(hz)`](https://defold.com/ref/stable/sys/?q=set_update_frequency#sys.set_update_frequency:frequency) to change this value at runtime.
+This setting also works in headless builds.
 
 #### Swap interval
 This integer value controls how the application deals with vsync. 0 disables vsync, and the default value is 1. When using an OpenGL adapter, this value sets the number of frames the window should [update between buffer swaps](https://www.khronos.org/opengl/wiki/Swap_Interval). For Vulkan, there is no built-in concept of swap interval, the value instead controls if vsync should be enabled or not.