Clarify the fixed indent align parameters message

Change the error message so it is clear that the alignment isn't with the
first parameter but with the line containing the first parameter with one
extra level of indentation.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#2983](https://github.com/bbatsov/rubocop/pull/2983): `Style/AlignParameters` message was clarified for `with_fixed_indentation` style. ([@dylanahsmith][])
+
 ## 0.39.0 (27/03/2016)
 
 ### New features
@@ -2082,3 +2086,4 @@
 [@necojackarc]: https://github.com/necojackarc
 [@laurelfan]: https://github.com/laurelfan
 [@amuino]: https://github.com/amuino
+[@dylanahsmith]: https://github.com/dylanahsmith

lib/rubocop/cop/style/align_parameters.rb

@@ -10,6 +10,12 @@ class AlignParameters < Cop
         include AutocorrectAlignment
         include OnMethodDef
 
+        ALIGN_PARAMS_MSG = 'Align the parameters of a method %s if they span ' \
+          'more than one line.'.freeze
+
+        FIXED_INDENT_MSG = 'Use one level of indentation for parameters ' \
+          'following the first line of a multi-line method %s.'.freeze
+
         def on_send(node)
           _receiver, method, *args = *node
 
@@ -27,8 +33,8 @@ def on_method_def(node, _method_name, args, _body)
 
         def message(node)
           type = node && node.parent.send_type? ? 'call' : 'definition'
-          "Align the parameters of a method #{type} if they span " \
-          'more than one line.'
+          msg = fixed_indentation? ? FIXED_INDENT_MSG : ALIGN_PARAMS_MSG
+          format(msg, type)
         end
 
         private

spec/rubocop/cop/style/align_parameters_spec.rb

@@ -504,7 +504,10 @@
                              '           b)',
                              'end'])
         expect(cop.offenses.size).to eq 1
-        expect(cop.offenses.first.to_s).to match(/method definition/)
+        expect(cop.messages)
+          .to eq(['Use one level of indentation for parameters ' \
+                  'following the first line of a multi-line method ' \
+                  'definition.'])
       end
 
       it 'registers an offense for parameters with double indent' do