Change guidance: add site-packages to Python path with site.addsitedir

Address #30844 by changing our guidance about Python site-packages. Right now, we tell users to write this .pth file to pick up the Homebrew site-packages directory with a foreign or system Python:

> /usr/local/lib/python2.7/site-packages

But we should be telling them to use:

> import site; site.addsitedir("/usr/local/lib/python2.7/site-packages")

which requests that Python follow .pth files in our site-packages. This is necessary to support wxpython and perhaps other formulæ and may help enable less fragile Python installation strategies.

This PR changes the python_caveats and also adds a doctor check. The doctor check will probably annoy people. :) I'd like to encourage existing installations to migrate because I have secret plans that need .pth files to work, but maybe I should wait until we need it to add the doctor check.
Not quite clear what this comment refers to.