Remove line about username and password not being supported in http_proxy URI

doc/environment_variables.md

@@ -5,8 +5,7 @@ gRPC C core based implementations (those contained in this repository) expose
 some configuration as environment variables that can be set.
 
 * http_proxy
-  The URI of the proxy to use for HTTP CONNECT support.  Does not currently
-  support username or password information in the URI.
+  The URI of the proxy to use for HTTP CONNECT support.
 
 * GRPC_ABORT_ON_LEAKS
   A debugging aid to cause a call to abort() when gRPC objects are leaked past