Improve "Setting Up a Mail Session for the Confluence Distribution" document

https://confluence.atlassian.com/doc/setting-up-a-mail-session-for-the-confluence-distribution-6328.html

There are many other parameters that can be added to the Resource block when configuring a mail session in Confluence. (i.e. "mail.smtp.ssl.enable")

This suggestion is to add a reference to the JavaMail documentation, which outlines the given parameters: https://javaee.github.io/javamail/docs/api/com/sun/mail/smtp/package-summary.html

The document could also mention that adding mail.debug="true" in the Resource block in server.xml will add debug messages to the Tomcat logs which can be helpful when troubleshooting Mail Server issues.