Update hooks should not say "Implements hook_update_N()." in the docblock.

Update hooks are an exception to the coding guideline that says the docblock should say which hook is implemented (see https://www.drupal.org/node/1354#updateN).

These use a different syntax, because the documentation summary (first line) is displayed to someone running update.php to tell them which updates need to run. So they are documented like this: [...]

This also means the description should be just a single line, which is nog always the case (Drush updb will actually show more than the first line, but update.php will not).