[XCConfigIntegrator] Update descriptions to be more verbose

Thanks @alloy!

lib/cocoapods/installer/user_project_integrator/target_integrator/xcconfig_integrator.rb

@@ -78,10 +78,12 @@ module Pod
             group = config.project['Pods'] || config.project.new_group('Pods')
             file_ref = group.files.find { |f| f.path == path }
             if config.base_configuration_reference != file_ref
-              UI.warn "CocoaPods did not set the base configuration of your " \
-              "project because one is already set. Please either set the " \
-              "base configurations of the target `#{target.name}` to " \
-              "`#{path}` or include the `#{path}` in your build configuration."
+              UI.warn 'CocoaPods did not set the base configuration of your ' \
+              'project because because your project already has a custom ' \
+              'config set. In order for CocoaPods integration to work at ' \
+              'all, please either set the base configurations of the target ' \
+              "#{target.name}` to `#{path}` or include the `#{path}` in your " \
+              'build configuration.'
             else
               file_ref ||= group.new_file(path)
               config.base_configuration_reference = file_ref

spec/unit/installer/user_project_integrator/target_integrator/xcconfig_integrator_spec.rb

@@ -48,8 +48,8 @@ module Pod
       config.base_configuration_reference.should.equal existing
     end
 
-    it 'does not set the Pods xcconfig as the base config if the base ' \
-       'config is already set' do
+    it 'does not set the Pods xcconfig as the base config if the user ' \
+       'has already set a config of their own' do
       sample_config = @project.new_file('SampleConfig.xcconfig')
       @target.build_configurations.each do |config|
         config.base_configuration_reference = sample_config