Document hook_commerce_license_period_info_alter

Problem/Motivation

Commerce license includes three license period plugins:

1. Unlimited
2. Rolling interval
3. Interval based on reference date

They can be modified (or removed) via the hook_ commerce_license_period_info_alter(), similar to hook_commerce_license_type_info_alter.

This should be documented in commerce_license.api.php.

Note: if you remove a plugin by unsetting it via this hook, ensure you do NOT have any data referencing this hook, or you will encounter a WSOD.