The definitions are scattered through the Arduino installation, but for example the register addresses you asked about used are defined under board-specific files under hardware\tools\avr\avr\include\avr.
They aren't documented as part of the Arduino runtime environment, I suppose on the basis that they aren't part of the API - they are part of the implementation. It's possible to bypass the Arduino library and access the hardware directly, and obviously various clever people have done that - and some of them have documented what they did and how it works - but those definitions are not documented as part of the API.
You mentioned the DATE and TIME preprocessor directives. There are some macros and directives which are defined by the C++ language, and others that are added by the GCC compiler that Arduino uses. These are also not part of the Arduino API and so are not covered by the Arduino documentation, but you're free to look up the language specification and compiler behaviour for yourself if it interests you.