doc: expand Source docstring

pipeline/utils.py

@@ -34,6 +34,24 @@ class Source():
         "path": "/path/to/video.mp4",
         "provider": "FileInputJSON"
     }
+
+    Instance variables:
+
+    source -- the source of the media file (eg, a URL or a local path)
+    path -- the path to the media file
+    provider -- the provider of the media file (eg, "FileInputJSON")
+
+    Accessing the object should return the path to the media file.
+
+    Methods:
+
+    duration() -- return the duration of the media file (uses ffprobe, result is cached)
+
+    Notes:
+     - source and path may be the same, for example in the case of a local file
+    """
+    _duration = None
+
     def __init__(self, source, path, provider):
         if not source:
             raise ValueError("Source must be provided") # TODO: #API -- decide if this is necessary