None of the placeholders have descriptions, and some are obvious but others arent. I'd also like to ask if there is a placeholder that will produce a heart if they are married and an empty string if they arent?

You can use the "%marriagemaster_heart%" placeholder by changing the NotMarried.Default string in the language file to an empty string (Default: ""), this will however change it for all placeholders.
Alternative you can check the v2.1 alpha which will allow to change formatting and messages for all placeholders individually in the language file.

The wiki page now has descriptions for all placeholders. It now also includes the new v2.1 placeholders and how the formatting of them can be changed.