doc: Explain that the default RTMP user agent is different when publishing

The fact that a different user agent is used is cruicial for getting
publishing authentication working. (When using librtmp, this other
user agent has to be specified manually, but that's not needed
with the libavformat internal RTMP support.)
Signed-off-by: Martin Storsjö <martin@martin.st>

doc/protocols.texi

@@ -226,7 +226,8 @@ times to construct arbitrary AMF sequences.
 
 @item rtmp_flashver
 Version of the Flash plugin used to run the SWF player. The default
-is LNX 9,0,124,2.
+is LNX 9,0,124,2. (When publishing, the default is FMLE/3.0 (compatible;
+<libavformat version>).)
 
 @item rtmp_flush_interval
 Number of packets flushed in the same request (RTMPT only). The default