[ruby/irb] Add an explanation of default sub commands of "measure", which are :time and :stackprof

https://github.com/ruby/irb/commit/759be5a344

lib/irb/cmd/measure.rb

@@ -9,6 +9,9 @@ module IRB
       end
 
       def execute(type = nil, arg = nil, &block)
+        # Please check IRB.init_config in lib/irb/init.rb that sets
+        # IRB.conf[:MEASURE_PROC] to register default "measure" methods,
+        # "measure :time" (abbreviated as "measure") and "measure :stackprof".
         case type
         when :off
           IRB.conf[:MEASURE] = nil