Remove http://DOCUMENTATION_PENDING/node/ links to Drupal 6 documentation pages

[avpaderno]

The documentation comment for form_example_tutorial_6_validate() contains the following sentence, for example.

The value report is $form_state['values'] (see http://DOCUMENTATION_PENDING/node/144132#form-state).

https://www.drupal.org/node/144132#form-state takes to _Drupal 5.x to 6.x FormAPI changes / FormAPI now uses the $formstate variable. which explains the differences between Drupal 5.x and Drupal 6.x. It is not information useful to Backdrop developers.

Those links can be removed.

[avpaderno]

This task is part of https://github.com/backdrop-contrib/examples/issues/23.