diff --git a/saharaclient/api/job_binaries.py b/saharaclient/api/job_binaries.py
index <HASH>..<HASH> 100644
--- a/saharaclient/api/job_binaries.py
+++ b/saharaclient/api/job_binaries.py
@@ -25,7 +25,14 @@ class JobBinariesManager(base.ResourceManager):
 
     def create(self, name, url, description=None, extra=None, is_public=None,
                is_protected=None):
-        """Create a Job Binary."""
+        """Create a Job Binary.
+
+        :param dict extra: authentication info needed for some job binaries,
+            containing the keys `username` and `password` for job binary in
+            Swift or the keys `accesskey`, `secretkey`, and `endpoint` for job
+            binary in S3
+
+        """
         data = {
             "name": name,
             "url": url
@@ -73,7 +80,9 @@ class JobBinariesManager(base.ResourceManager):
         * url
         * is_public
         * is_protected
-        * extra - dict with `user` and `password` keyword arguments
+        * extra - dict with the keys `username` and `password` for job binary
+          in Swift, or with the keys `accesskey`, `secretkey`, and `endpoint`
+          for job binary in S3
         """
         return self._update(
             '/job-binaries/%s' % job_binary_id, data, 'job_binary')
Properly document job binary "extra"

After bp sahara-support-s3 the use of the "extra" parameter is not only
for binaries stored in Swift.

So, modify some docstrings (which are in turn included in the actual
docs) to reflect this.

Change-Id: I<I>fce<I>a7d<I>d<I>f<I>e9a8b1d8ebb6ec1