Part 1: Docs cleanup to MenuLinkManagerInterface
Part 1: adding the doc for an exception on createLink if the link already exists. Implementation of the exception in a future commit.
Part 1: clean up of MenuLinkManagerInterface
custom menu links text fix
START PART 2:  https://www.drupal.org/node/2301273
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkTree-review-do-not-test.patch
START PART 3: https://www.drupal.org/node/2301313
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkContentUI-review-do-not-test.patch