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.
In the case that:

1. the formula is not keg-only,
1. system Python hasn't been configured to read brewed .pth files,
1. system Python has been configured to look at Homebrew's site-packages, and
1. Python modules have been installed but .pth files have not,

this allows "nil" to be returned (because importing the installed modules will work fine).  I could equivalently add an "else nil" block to the end of the final if/elsif.