diff --git a/README.md b/README.md
index 55fbd98..fbc67a1 100644
--- a/README.md
+++ b/README.md
@@ -41,7 +41,7 @@ Optional arguments:
 
 Optional arguments:
 
-- `repository_url` (default `https://pypi.python.org/pypi/%s/json`): format string to use a different repository pypi repository URL. For example test.pypi.org or a private repository. Formatter is passed the package name as the first argument.
+- `repository_url` (default `https://pypi.python.org/pypi/%s/json`): a `%` style format string to use a different repository PyPI repository URL, e.g. test.pypi.org or a private repository. The string is formatted with the package name.
 
 This still makes the HTTP call with retries and caches the result on disk. It doesn't use a separate thread or emit any warnings (unless there is an exception specifically while using the cache, in which case the check will be done without the cache).
 
diff --git a/outdated/__init__.py b/outdated/__init__.py
index 3593e01..72f698c 100644
--- a/outdated/__init__.py
+++ b/outdated/__init__.py
@@ -21,6 +21,11 @@ def check_outdated(package, version, repository_url='https://pypi.python.org/pyp
     Attempts to cache on disk the HTTP call it makes for 24 hours. If this
     somehow fails the exception is converted to a warning (OutdatedCacheFailedWarning)
     and the function continues normally.
+
+    `repository_url` is a `%` style format string
+    to use a different repository PyPI repository URL,
+    e.g. test.pypi.org or a private repository.
+    The string is formatted with the package name.
     """
 
     from pkg_resources import parse_version