core: Add docstring to is_blacklisted_import()

This documents the existing behaviour, which may not be the intended.

mitogen/core.py

@@ -168,6 +168,14 @@ def restart(func, *args):
 
 
 def is_blacklisted_import(importer, fullname):
+    """Return ``True`` if `fullname` is part of a blacklisted package, or if
+    any packages have been whitelisted and `fullname` is not part of one.
+
+    NB:
+      - If a package is on both lists, then it is treated as blacklisted.
+      - If any package is whitelisted, then all non-whitelisted packages are
+        treated as blacklisted.
+    """
     return ((not any(fullname.startswith(s) for s in importer.whitelist)) or
                  (any(fullname.startswith(s) for s in importer.blacklist)))