Docs: warn users about `astropy.test()` from within repository.

The most prominent instructions for testing astropy on the installation page say to do

    >>> import astropy
    >>> astropy.test()

However, if this is done from the root directory of the source repository, the source `astropy` is used rather than the built/installed version, and many tests fail as a result. We should add a note to the installation page explicitly warning about this.