Add comment to explain why CLOCK_MONOTONIC comes first

Author: tomstuart

src/main/ruby/truffleruby/core/process.rb

@@ -156,7 +156,7 @@ def self.clock_gettime(id, unit=:float_second)
     end
 
     case id
-    when CLOCK_MONOTONIC
+    when CLOCK_MONOTONIC # most common clock id
       time = Primitive.process_time_nanotime
     when CLOCK_REALTIME
       time = Primitive.process_time_instant