add docstring

scrapy/pqueues.py

@@ -30,6 +30,25 @@ def _path_safe(text):
 
 
 class ScrapyPriorityQueue(object):
+    """A priority queue implemented using multiple internal queues (typically,
+    FIFO queues). The internal queue must implement the following methods:
+
+        * push(obj)
+        * pop()
+        * close()
+        * __len__()
+
+    The constructor receives a downstream_queue_cls argument, which is a class
+    used to instantiate a new (internal) queue when a new priority is allocated.
+
+    Only integer priorities should be used. Lower numbers are higher
+    priorities.
+
+    startprios is a sequence of priorities to start with. If the queue was
+    previously closed leaving some priority buckets non-empty, those priorities
+    should be passed in startprios.
+
+    """
 
     @classmethod
     def from_crawler(cls, crawler, downstream_queue_cls, key, startprios=()):