update docs for settings

docs/topics/jobs.rst

@@ -96,6 +96,8 @@ But this will::
         somearg = response.meta['somearg']
         print "the argument passed is:", somearg
 
-If you wish to log the requests that couldn't be serialized, you can set the ``SCHEDULER_DEBUG`` setting to ``True`` in the project's settings page. It is ``False`` by default.
+If you wish to log the requests that couldn't be serialized, you can set the
+``SCHEDULER_DEBUG`` setting to ``True`` in the project's settings page.
+It is ``False`` by default.
 
 .. _pickle: http://docs.python.org/library/pickle.html

docs/topics/settings.rst

@@ -1033,8 +1033,15 @@ SCHEDULER_DEBUG
 
 Default: ``False``
 
-Setting to ``True`` will log the first unserializable request encountered.
-Stats collected.
+Setting to ``True`` will log debug information about the requests scheduler.
+This currently logs(only once) if the requests cannot be serialized to disk.
+Stats counter (``scheduler/unserializable``) tracks the number of times this happens.
+
+Example entry in logs::
+
+    1956-01-31 00:00:00+0800 [scrapy] ERROR: Unable to serialize request: <request>
+    - reason: <exception> - no more unserializable requests will be logged (stats being
+    collected)
 
 .. setting:: SPIDER_CONTRACTS
 

scrapy/core/scheduler.py

@@ -84,15 +84,16 @@ class Scheduler(object):
         try:
             reqd = request_to_dict(request, self.spider)
             self.dqs.push(reqd, -request.priority)
-        except ValueError as e: # non serializable request
+        except ValueError as e:  # non serializable request
             if self.logunser:
-                msg = ("Unable to serialize request: %(request)s - reason: %(reason)s"
-                       " - no more unserializable requests will be logged"
-                       " (stats being collected)")
+                msg = ("Unable to serialize request: %(request)s - reason:"
+                       " %(reason)s - no more unserializable requests will be"
+                       " logged (stats being collected)")
                 logger.error(msg, {'request': request, 'reason': e},
                              exc_info=True, extra={'spider': self.spider})
                 self.logunser = False
-            self.stats.inc_value('scheduler/unserializable', spider=self.spider)
+            self.stats.inc_value('scheduler/unserializable',
+                                 spider=self.spider)
             return
         else:
             return True