Add comment for LiteralExpander select (#12926)

Co-authored-by: Beta Ziliani <[email protected]>

src/compiler/crystal/semantic/literal_expander.cr

@@ -631,6 +631,48 @@ module Crystal
       final_exp
     end
 
+    # Convert a `select` statement into a `case` statement based on `Channel.select`
+    #
+    # From:
+    #
+    #     select
+    #     when foo then body
+    #     when x = bar then x.baz
+    #     end
+    #
+    # To:
+    #
+    #     %index, %value = ::Channel.select({foo_select_action, bar_select_action})
+    #     case %index
+    #     when 0
+    #       body
+    #     when 1
+    #       x = value.as(typeof(foo))
+    #       x.baz
+    #     else
+    #       ::raise("BUG: invalid select index")
+    #     end
+    #
+    #
+    # If there's an `else` branch, use `Channel.non_blocking_select`.
+    #
+    # From:
+    #
+    #     select
+    #     when foo then body
+    #     else qux
+    #     end
+    #
+    # To:
+    #
+    #     %index, %value = ::Channel.non_blocking_select({foo_select_action})
+    #     case %index
+    #     when 0
+    #       body
+    #     else
+    #       qux
+    #     end
+    #
     def expand(node : Select)
       index_name = @program.new_temp_var_name
       value_name = @program.new_temp_var_name