I find the reference documentation at Arduino Reference - Arduino Reference pretty lacking and it would help a lot if there were links to the source for the objects/methods discussed. Then at least I could actually figure out what all the interfaces are for the libraries. I'm sure I could search google for the source but it should be standard practice to reference the source in the documentation!
My 2 cents.
A tricky thing about this is that, even though the API is standardized, many of these are implemented differently by each board platform. So it would be necessary to provide a link to the source code in each of the growing number of official platforms. Of course that would never include the dozens of 3rd party platforms.
Some of the functions are architecture independent. Traditionally, these were always duplicated in every platform, and you might find that an improvement or fix in one platform hadn't been ported to another. To help resolve this situation, Arduino recently moved all the architecture independent code to a dedicated repository, from which it can be shared by all platforms:
However, this is currently only in use by two official platforms and probably only a few 3rd party platforms, so it would be incorrect to point people to that code alone.
Another issue is that the code is under continuous development. So if you want to see the code that is actually running on your computer, you need to look at that version of the source code. So it might make more sense to look at the files on your computer, rather than the ones online. The alpha stage Arduino Pro IDE does have the capability to display the definition or declaration for any code, including that of the Arduino core library.