Mention that overlay requires a certain HeadsetFeature;

api/init.lua

@@ -189,7 +189,7 @@ return {
                     {
                       name = "overlay",
                       type = "boolean",
-                      description = "Whether the project should run as an overlay.  Can also be a number to control sort order against other overlays (default is zero, higher numbers go on top)."
+                      description = "Whether the project should run as an overlay.  Can also be a number to control sort order against other overlays (default is zero, higher numbers go on top).  Requires the `overlay` headset feature to be supported, see `lovr.headset.getFeatures`."
                     }
                   }
                 },

api/lovr/callbacks/conf.lua

@@ -193,7 +193,8 @@ return {
               type = 'boolean',
               description = [[
                 Whether the project should run as an overlay.  Can also be a number to control sort
-                order against other overlays (default is zero, higher numbers go on top).
+                order against other overlays (default is zero, higher numbers go on top).  Requires
+                the `overlay` headset feature to be supported, see `lovr.headset.getFeatures`.
               ]]
             }
           }