Yes, definitely, documenting your project is absolutely necessary. But I'm not talking about for automation tests specifically, but for all programs in general.
Selenium automation tests are programs just like anything else, which means that all coding standards within your company should also apply. The quality of the automation tests should be as high as other code bases.
It depends on how your company defines the coding guidelines. As far as I know, some companies discourage the excess usage of comments, because the code itself should be self-explanatory.
For example, all method/variable names should be in some kind of pre-defined format, usages of semi-colon, brackets should be consistent company-wide, no long methods, no deeply nested control flows etc. All Selenium code should comply with the company rules completely.
In your situation, if you can't figure what a class does, then the problem is not about lack of commenting, but more about a poorly designed/named class. In that case, if your Selenium project is extremely messy and complex, I'd suggest refactor the relevant classes to make the project smell better, not simply add comments.
In my opinion, you don't need to find resources on documenting automation tests specifically, but for programming in general. For instance, Clean Code by Robert C. Martin would be a good book to start off.