Add documentation about reserved words in pattern definitions

[aleksip]

Not 100 % sure but I don't think the documentation mentions anything about reserved words in pattern definitions?

  /**
   * Reserved words.
   *
   * @var array
   */
  protected $reserved = [
    'id',
    'type',
    'theme',
    'context',
    'use',
    'attributes',
  ];

It would be good to document them as the resulting error message is a bit cryptic.