Restored missing threads source flag (syslog-ng#100)

The "threaded()" source flag was lost during conversation, now it is
restored.

_includes/doc/admin-guide/options/source-flags.md

@@ -1,6 +1,6 @@
 ## flags()
 
-|Type:| assume-utf8, empty-lines, expect-hostname, kernel, no-hostname, no-multi-line, no-parse, sanitize-utf8, store-legacy-msghdr, store-raw-message, syslog-protocol, validate-utf8|
+|Type:| assume-utf8, empty-lines, expect-hostname, kernel, no-hostname, no-multi-line, no-parse, sanitize-utf8, store-legacy-msghdr, store-raw-message, syslog-protocol, threaded, validate-utf8|
 |Default: |  empty set|
 
 *Description:* Specifies the log parsing options of the source.
@@ -113,6 +113,14 @@
     Note that this flag is not needed for the syslog driver, which
     handles only messages that have a frame header.
 
+- *threaded*: The threaded flag enables multithreading for the
+    destination. For details on multithreading, see
+    Multithreading and scaling in {{ site.product.short_name }}.
+
+    **NOTE:** The file destination uses multiple threads only if the
+    destination filename contains macros.
+    {: .notice--info}
+
 - *validate-utf8*: The validate-utf8 flag enables
     encoding-verification for messages formatted according to the new
     IETF syslog standard (for details, see