plugin info alter hook needs to be documented in api.php file

Problem/Motivation

The plugin manager invents the alter hook data_provider_info.

This needs to be documented in an api.php file.

Steps to reproduce

Proposed resolution

Add the api.php file.

Tagging as novice -- see hook_filter_info_alter() or hook_media_source_info_alter() in core for examples.

Remaining tasks

User interface changes

API changes

Data model changes