update doc

doc/environment_variables.md

@@ -54,12 +54,14 @@ some configuration as environment variables that can be set.
   - op_failure - traces error information when failure is pushed onto a
     completion queue
   - round_robin - traces the round_robin load balancing policy
+  - resource_quota - trace resource quota objects internals
   - glb - traces the grpclb load balancer
   - queue_pluck
   - queue_timeout
   - server_channel - lightweight trace of significant server channel events
   - secure_endpoint - traces bytes flowing through encrypted channels
   - timer - timers (alarms) in the grpc internals
+  - timer_check - more detailed trace of timer logic in grpc internals
   - transport_security - traces metadata about secure channel establishment
   - tcp - traces bytes in and out of a channel