Refine offense message for Style/WhenThen

This PR refines offense message for `Style/WhenThen`.
It uses actual value of `when` for offense message.

lib/rubocop/cop/style/when_then.rb

@@ -20,12 +20,16 @@ module Style
       class WhenThen < Base
         extend AutoCorrector
 
-        MSG = 'Do not use `when x;`. Use `when x then` instead.'
+        MSG = 'Do not use `when %<expression>s;`. Use `when %<expression>s then` instead.'
 
         def on_when(node)
           return if node.multiline? || node.then? || !node.body
 
-          add_offense(node.loc.begin) { |corrector| corrector.replace(node.loc.begin, ' then') }
+          message = format(MSG, expression: node.conditions.map(&:source).join(', '))
+
+          add_offense(node.loc.begin, message: message) do |corrector|
+            corrector.replace(node.loc.begin, ' then')
+          end
         end
       end
     end

spec/rubocop/cop/style/when_then_spec.rb

@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::Cop::Style::WhenThen, :config do
-  it 'registers an offense for when x;' do
+  it 'registers an offense for when b;' do
     expect_offense(<<~RUBY)
       case a
       when b; c
-            ^ Do not use `when x;`. Use `when x then` instead.
+            ^ Do not use `when b;`. Use `when b then` instead.
       end
     RUBY
 
@@ -16,6 +16,21 @@
     RUBY
   end
 
+  it 'registers an offense for when b, c;' do
+    expect_offense(<<~RUBY)
+      case a
+      when b, c; d
+               ^ Do not use `when b, c;`. Use `when b, c then` instead.
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      case a
+      when b, c then d
+      end
+    RUBY
+  end
+
   it 'accepts ; separating statements in the body of when' do
     expect_no_offenses(<<~RUBY)
       case a