You can create documentation about BAdIs at the following places:
Definition/Implementation Side
Type of Documentation
Definition Side
Documentation of the relevant enhancement spot
Documentation of the BAdI itself:
Externally visible documentation - this documentation can be stored, for example, in the IMG. The documentation has to describe the role and purpose of the BAdI. The methods of the BAdI must not be described here, but in the BAdI interface instead.
Technical documentation - this documentation describes the BAdI from a technical point of view. It has to contain information that supports BAdI developers with any further developments or error handling.
Documentation of the BAdI interface and its components
Documentation of the default and example implementation classes