Pre-built PHPMailer API documentation is not provided with the PHP classes, but you can generate API documentation by running
phpdoc in the top-level folder of this project, and documentation will be generated in this folder. You will need to have phpDocumentor installed, though you will already have that if you installed PHPMailer using composer with dev modules enabled. This documentation is also available online, though it may not be quite up to date or match your version exactly. The configuration for phpdoc is in the phpdoc.dist.xml file.
Fixes and additions to documentation are welcome - please submit pull requests or improve wiki pages.