Update documentation api links to point to D8 version

This looks good, and I verified that all the URLs work as expected:

-    'website' => 'http://api.drupal.org/api/drupal/includes--ajax.inc/group/ajax/7',
+    'website' => 'http://api.drupal.org/api/drupal/core%21includes%21ajax.inc/group/ajax/8',

Would it be cleaner (and a bit more future-proof) to use http://api.drupal.org/api/group/ajax/8 here instead? It seems to go to the same place.

#2 - yes!

To point to the Form API reference, just use the file name forms_api_reference.html instead of the URL.

To point to a topic, just use the topic's machine name, such as "ajax", instead of the URL.

If there are any other places in core that have links to api.drupal.org, they should all be removed.

Sorry, to clarify: #3 only applies to documentation headers. If there are links being made in Help, then they need to have the URL, and the URL suffix needs to have /8 on it to be "future-proof".

And for URLs with #suffix on them, I would reword so that the on-page reference isn't needed. Just say "See the @link forms_api_reference.html Form API reference @endlink for more information." or something like that.

Mostly good! A couple of things to fix:

a) In the first hunk in the patch, leave off the /8 -- just put in the file name:

+ *   More information is available in the @link forms_api_reference.html/8 Form
+ *   API Reference @endlink

Also, @link ... @endlink should always be all on one line. If that makes it go over 80 characters, start the @link on a new line.

b)

+++ b/core/modules/field/field.api.php
@@ -691,7 +691,7 @@ function hook_field_is_empty($item, $field) {
  * hook_field_widget_info().
  *
  * Widgets are
- * @link http://api.drupal.org/api/drupal/developer--topics--forms_api_reference.html Form API @endlink
+ * @link forms_api_reference.html Form API @endlink

I think that @link...@endlink can be moved up to the previous line and not exceed 80 characters? If not, some of the words from the next line can be moved up.

c) It would be good if someone could check all the replaced URLs and make sure they all go to the correct place. Many of them redirect, which is probably OK (the URLs in the files are simpler than the ones they are redirected to), but someone should test them. I didn't do that yet.

(c) - check the URLs == copy the URL from the patch, paste it into your browser's URL bar, and verify it goes to the correct function/class/topic documentation as intended. :)

Looks good! This touches quite a few files, so I'll leave it at RTBC for a few days, and probably commit on Friday the 4th, if there are no issues tagged "avoid commit conflicts" that conflict at that time.

Thanks! This has been committed to 8.x.