Docs: clarify the handling of exceptions raised in errbacks (#7898)

docs/topics/request-response.rst

@@ -770,6 +770,10 @@ is raise while processing it.
 It receives a :exc:`~twisted.python.failure.Failure` as first parameter and can
 be used to track connection establishment timeouts, DNS errors etc.
 
+If an errback raises an exception, Scrapy logs it and sends the
+:signal:`spider_error` signal, unless the exception is the one that the errback
+received, which Scrapy logs as a download error instead.
+
 Here's an example spider logging all errors and catching some specific
 errors if needed:
 

docs/topics/spider-middleware.rst

@@ -145,8 +145,9 @@ one or more of these methods:
 
     .. method:: process_spider_exception(response, exception)
 
-        This method is called when a spider or :meth:`process_spider_output`
-        method (from a previous spider middleware) raises an exception.
+        This method is called when a spider callback or a
+        :meth:`process_spider_output` method (from a previous spider
+        middleware) raises an exception.
 
         :meth:`process_spider_exception` should return either ``None`` or an
         iterable of :class:`~scrapy.Request` or :ref:`item <topics-items>`