Merge pull request #1979 from rubocop/fix-wait

Fix problem with `Open3.popen3` where `confirm_documentation` task never finishes waiting for child processes

Rakefile

@@ -72,12 +72,15 @@ end
 
 desc 'Confirm documentation is up to date'
 task confirm_documentation: :generate_cops_documentation do
-  _, _, _, process =
-    Open3.popen3('git diff --exit-code docs/')
-
-  unless process.value.success?
-    raise 'Please run `rake generate_cops_documentation` ' \
-          'and add docs/ to the commit.'
+  stdout, _stderr, status =
+    Open3.capture3('git diff --exit-code docs/')
+
+  unless status.success?
+    warn 'Documentation is out of sync:'
+    warn stdout
+    warn 'Please run `rake generate_cops_documentation` ' \
+         'and add docs/ to the commit.'
+    exit 1
   end
 end