Setting documentation.

docs/topics/settings.rst

@@ -742,6 +742,17 @@ If :setting:`DOWNLOAD_DELAY` is zero (default) this option has no effect.
 
 .. _wget: http://www.gnu.org/software/wget/manual/wget.html
 
+.. setting:: REACTOR_THREADPOOL_MAXSIZE
+
+REACTOR_THREADPOOL_MAXSIZE
+------------------
+
+Default: ``10``
+
+The maximum limit for Twisted Reactor thread pool size. This is common multi-purpose thread pool used by various
+Scrapy components. Threaded DNS Resolver, BlockingFeedStorage, S3FilesStore just to name a few. Increase this value if
+you're experiencing problems with insufficient blocking IO.
+
 .. setting:: REDIRECT_MAX_TIMES
 
 REDIRECT_MAX_TIMES