diff --git a/django/contrib/auth/base_user.py b/django/contrib/auth/base_user.py
index 0f8d5d87e2..357528e19e 100644
--- a/django/contrib/auth/base_user.py
+++ b/django/contrib/auth/base_user.py
@@ -115,6 +115,11 @@ class AbstractBaseUser(models.Model):
         self.password = make_password(None)
 
     def has_usable_password(self):
+        """
+        Return False if set_unusable_password() has been called for this user,
+        or if the password is None, or if the password uses a hasher that's not
+        in the PASSWORD_HASHERS setting.
+        """
         return is_password_usable(self.password)
 
     def get_session_auth_hash(self):
diff --git a/docs/ref/contrib/auth.txt b/docs/ref/contrib/auth.txt
index b3fca78d73..e05280367c 100644
--- a/docs/ref/contrib/auth.txt
+++ b/docs/ref/contrib/auth.txt
@@ -212,7 +212,9 @@ Methods
 
         Returns ``False`` if
         :meth:`~django.contrib.auth.models.User.set_unusable_password()` has
-        been called for this user.
+        been called for this user, or if the password is ``None``, or if the
+        password uses a hasher that's not in the :setting:`PASSWORD_HASHERS`
+        setting.
 
     .. method:: get_group_permissions(obj=None)