Add note to docs about limitation of packages_distributions. Fixes #402.

Author: jaraco

docs/using.rst

@@ -289,6 +289,10 @@ Python module or :term:`packaging:Import Package`::
     >>> packages_distributions()
     {'importlib_metadata': ['importlib-metadata'], 'yaml': ['PyYAML'], 'jaraco': ['jaraco.classes', 'jaraco.functools'], ...}
 
+Some editable installs, `do not supply top-level names
+<https://github.com/pypa/packaging-problems/issues/609>`_, and thus this
+function is not reliable with such installs.
+
 .. _distributions:
 
 Distributions