Add note about asynchronous headset rendering;

api/init.lua

@@ -14291,7 +14291,8 @@ return {
               },
               returns = {}
             }
-          }
+          },
+          notes = "On the `web` and `android` platforms (see `lovr.getOS`), headset rendering is asynchronous and the callback passed to `lovr.headset.renderTo` will not be called immediately.  This is due to certain restrictions inherent to these platforms."
         },
         {
           name = "setClipDistance",

api/lovr/headset/renderTo.lua

@@ -23,5 +23,10 @@ return {
       ]]
     }
   },
-  returns = {}
+  returns = {},
+  notes = [[
+    On the `web` and `android` platforms (see `lovr.getOS`), headset rendering is asynchronous and
+    the callback passed to `lovr.headset.renderTo` will not be called immediately.  This is due to
+    certain restrictions inherent to these platforms.
+  ]]
 }