[DOC] The Document-method hint is needed only in C files

'Document-method' string in C file has a special treatment. In Ruby source file,
however, it is considered to be part of the documentation.

Note Numeric#size was migrated from C to Ruby, in 3208a5df2d

numeric.rb

@@ -210,8 +210,6 @@ class Integer
     self
   end
 
-  #
-  #  Document-method: Integer#size
   #  call-seq:
   #     int.size  ->  int
   #