Fix various defects in PHPDoc in-code documentation

In various places, there are minor and major defects in the PHPDoc in-code documentation. Ranging from spelling/grammar errors to wholly incorrect descriptions, they can be confusing or misleading. Fixing them will help anyone new to the code or refreshing his/her memory to understand the workings more easily.

I'm creating this issue to address some errors in documentation on events and the notification system, but I propose to make this a general ticket any pull request fixing PHPDoc blocks can refer to.