[ruby/yarp] More documentation for desugar compiler

https://github.com/ruby/yarp/commit/4a6df3c2a4

lib/yarp/desugar_compiler.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 module YARP
+  # DesugarCompiler is a compiler that desugars Ruby code into a more primitive
+  # form. This is useful for consumers that want to deal with fewer node types.
   class DesugarCompiler < MutationCompiler
     # @@foo &&= bar
     #