Explain when mailingID and messageTemplateID are passed through

Linked to https://github.com/civicrm/civicrm-core/pull/15306

docs/hooks/hook_civicrm_alterMailContent.md

@@ -15,7 +15,13 @@ tokenizing it.
 
 ## Details
 
--   $content - fields include: html, text, subject, groupName, valueName, messageTemplateID
+$content - fields include: html, text, subject, groupName, valueName, messageTemplateID, mailingID
+Note that this hook is fired when: 
+- creating mailings through the traditional BAO mailer (standard CiviMail)
+- creating mailings through FlexMailer (used by Mosaico)
+- sending emails using message templates, in CRM_Core_BAO_MessageTemplate
+
+In the latter case there is inherently no mailingID, so this will not be supplied. Similarly in the 2 former cases the messageTemplateID is not supplied.
 
 ## Example