rtmp: Detect and warn if the user tries to pass librtmp style parameters

Signed-off-by: Martin Storsjö <martin@martin.st>
doc: Extend the rtmp example to include how to pass username/password

Signed-off-by: Martin Storsjö <martin@martin.st>
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: Add librtmp to the section header for the librtmp specific details

Signed-off-by: Martin Storsjö <martin@martin.st>
doc: Add an example on publishing over RTMP

Signed-off-by: Martin Storsjö <martin@martin.st>
doc: Clarify the avconv section about -re

Signed-off-by: Martin Storsjö <martin@martin.st>