document layer suspension config and add to whitelist (#3123)

config.js

     // Enable / disable simulcast support.
     // disableSimulcast: false,
 
+    // Enable / disable layer suspension.  If enabled, endpoints whose HD
+    // layers are not in use will be suspended (no longer sent) until they
+    // are requested again.
+    // enableLayerSuspension: false,
+
     // Suspend sending video if bandwidth estimation is too low. This may cause
     // problems with audio playback. Disabled until these are fixed.
     disableSuspendVideo: true,

react/features/base/config/functions.js

     'disableSuspendVideo',
     'displayJids',
     'enableDisplayNameInStats',
+    'enableLayerSuspension',
     'enableLipSync',
     'enableLocalVideoFlip',
     'enableRemb',