Include Warning About Relay Restrictions e.g. Google Gmail

[nbuuck]

A recurring theme in issues opened in ossec/ossec-hids is users attempting to relay their alerts through Gmail without conforming to Google's requirements. A generic warning somewhere beneath https://ossec.github.io/docs/manual/output/email-output.html or one of the pages it links to about these restrictions could help stymie these recurring issues.

As another example, Exchange Online has specific encryption and authentication requirements depending on the source and destination domains: https://support.office.com/en-us/article/How-to-set-up-a-multifunction-device-or-application-to-send-email-using-Office-365-69f58e99-c550-4274-ad18-c805d654b4c4