Document hook_mailchimp_campaign_alter()

This appears to be documented as of ✨ Allow module to interact with mailchimp campagin content Fixed . Reopen if I am missing something!

That commit documents hook_mailchimp_campaign_content_alter(), but there is another hook available called hook_mailchimp_campaign_alter() on line 85 of the mailchimp_campaign.module inside the mailchimp_campaign_save_campaign() function on the latest 3.x branch. That hook is not documented.

Ah, right on. Thanks!