improve error message for multiple documents

The YAML parser doesn't support multiple documents. This pull requests
improves the error message when the parser detects multiple YAML
documents.
Merge branch '2.3' into 2.5

* 2.3:
  added missing files
  [TwigBundle] added a test
  Indicate which file was being parsed if an exception is thrown while running translation:debug
  [ClassLoader] Cast $useIncludePath property to boolean
  [HttpFoundation] Minor spelling fix in PHPDocs
  improve error message for multiple documents
  [Session] remove invalid workaround in session regenerate
  [Kernel] ensure session is saved before sending response
  [Routing] serialize the compiled route to speed things up
  [Validator] Fixed Regex::getHtmlPattern() to work with complex and negated patterns
  [DependencyInjection] use inheritdoc for loaders
  [Config] fix filelocator with empty name
  [Form] fix form handling with unconventional request methods like OPTIONS
  CSRF warning docs on Request::enableHttpMethodParameterOverride()

Conflicts:
	src/Symfony/Component/Routing/Route.php