Additional documentation files have always been problematic and often ignored or placed in obscure places.
We considered placing them in the same directory as the source files for the packages, but we felt it was important that users be able to find documentation separately from sources, since most users have no interest in sources.
We hope that a separate, but parallel, structure for documentation would (1) keep the documentation together and (2) make it as straightforward as possible for users to find the particular documentation they were after.