I guess that is because pip is not part of the official Python. Maybe best described as a front-end to distutils (which is what your link documents and is part of official Python).

edit: Python docs front-page[1] also notes following:

A new documentation project, which will be merged into the Python documentation soon, covers creating, installing and distributing Python packages: http://guide.python-distribute.org/

[1] http://www.python.org/doc/

So I guess that they have acknowledged that the docs are suboptimal currently in this part.