Use fully-qualified class names in code in api.php files

Problem/Motivation

In vendor/drupal/coder/coder_sniffer/Drupal/Sniffs/Classes/FullyQualifiedNamespaceSniff.php we do

        // Skip this sniff in *api.php files because they want to have fully
        // qualified names for documentation purposes.
        if (substr($phpcsFile->getFilename(), -8) === '.api.php') {
            return ($phpcsFile->numTokens + 1);
        }

This should be formalized as a documentation standard.

Benefits

If we adopted this change, the Drupal Project would benefit by ...

Three supporters required

1. https://www.drupal.org/u/{userid} (date that user added support)
2. https://www.drupal.org/u/{userid} (date that user added support)
3. https://www.drupal.org/u/{userid} (date that user added support)

Proposed changes

Provide all proposed changes to the Drupal Coding standards. Give a link to each section that will be changed, and show the current text and proposed text as in the following layout:

1. {link to the documentation heading that is to change}

Current text

Add current text in blockquotes

Proposed text

Add proposed text in blockquotes

2. Repeat the above for each page or sub-page that needs to be changed.

Remaining tasks

1. Create this issue in the Coding Standards queue, using the defined template
2. Add supporters
3. Create a Change Record
4. Review by the Coding Standards Committee
5. Coding Standards Committee takes action as required
6. Tagged with 'Needs documentation edits' if Core is not affected
7. Discussed by the Core Committer Committee, if it impacts Drupal Core
8. Documentation updates
   1. Edit all pages
   2. Publish change record
   3. Remove 'Needs documentation edits' tag
9. If applicable, create follow-up issues for PHPCS rules/sniffs changes

For a full explanation of these steps see the Coding Standards project page