Document hook_storychief_field_handler_info_alter()

Problem/Motivation

The hook_storychief_field_handler_info_alter() hook can be used to override existing plugins, e.g. to create custom langcode mappings or to use a different text format for the body field. This hook isn't documented in storychief.api.php.

Proposed resolution

Document this hook in storychief.api.php.