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.
Further help and information is available in the PHPMailer README, the examples folder, and in the GitHub wiki.
Fixes and additions to documentation are welcome - please submit pull requests or improve wiki pages.