bpo-41376: Fix the documentation of site.getusersitepackages() (GH-21602)

`site.getusersitepackages()` returns the location of the user-specific site-packages directory
even when the user-specific site-packages is disabled.

```
$ python -s -m site
sys.path = [
    '/home/user/conda/lib/python37.zip',
    '/home/user/conda/lib/python3.7',
    '/home/user/conda/lib/python3.7/lib-dynload',
    '/home/user/conda/lib/python3.7/site-packages',
]
USER_BASE: '/home/user/.local' (exists)
USER_SITE: '/home/user/.local/lib/python3.7/site-packages' (doesn't exist)
ENABLE_USER_SITE: False
```

It was not practical to prevent the function from returning None if user-specific site-packages are disabled, since there are other uses of the function which are relying on this behaviour (e.g. `python -m site`).

Author: pelson

Doc/library/site.rst

@@ -231,7 +231,9 @@ Module contents
 
    Return the path of the user-specific site-packages directory,
    :data:`USER_SITE`.  If it is not initialized yet, this function will also set
-   it, respecting :envvar:`PYTHONNOUSERSITE` and :data:`USER_BASE`.
+   it, respecting :data:`USER_BASE`.  To determine if the user-specific
+   site-packages was added to ``sys.path`` :data:`ENABLE_USER_SITE` should be
+   used.
 
    .. versionadded:: 3.2