Merge pull request #703 from bhahumanists/mailingID-explanation

Explain when mailingID and messageTemplateID are passed through

docs/hooks/hook_civicrm_alterMailContent.md

@@ -15,7 +15,14 @@ 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