Document that all maximum values must be specified. (#181)

Fixes #179.

Author: g-easy

src/opencensus/proto/trace/v1/trace_config.proto

@@ -22,7 +22,8 @@ option java_outer_classname = "TraceConfigProto";
 
 option go_package = "github.com/census-instrumentation/opencensus-proto/gen-go/trace/v1";
 
-// Global configuration of the trace service.
+// Global configuration of the trace service. All fields must be specified, or
+// the default (zero) values will be used for each type.
 message TraceConfig {
 
   // The global default sampler used to make decisions on span sampling.