Add docstring to is_import_exception

Clarify what is_import_exception does. Leave the name as
is_import_exception and no is_import_whitelisted  because
import-exceptions is part of the API for hacking (used in the hacking
section of tox.ini).

Change-Id: Ib36d84078ecfcdbcb62137b40ab7426738e56a45
Merge "Add docstring to is_import_exception"