more accurate description in environment_variables.md

doc/environment_variables.md

@@ -49,7 +49,9 @@ some configuration as environment variables that can be set.
   - connectivity_state - traces connectivity state changes to channels
   - channel_stack_builder - traces information about channel stacks being built
   - executor - traces grpc's internal thread pool ('the executor')
-  - fd_trace - traces fd creation, shutdown and closure.
+  - fd_trace - traces fd create(), shutdown() and close() calls for channel fds.
+    Also traces epoll fd create()/close() calls in epollex polling engine
+    traces epoll-fd creation/close calls for epollex polling engine
   - glb - traces the grpclb load balancer
   - handshaker - traces handshaking state
   - http - traces state in the http2 transport engine