rtph26[45]pay: Change default aggregate-mode to "none" for backwards compatibility

We didn't aggregate at all in previous versions and there are apparently
various RTP implementations that don't handle aggregation well at all.

As part of this also document that for RTSP it is recommended to keep it
set to "none" while for WebRTC it should be set to "zero-latency".

Fixes gstreamer/gst-plugins-good#749

Part-of: <gstreamer/gst-plugins-good!692>
31 jobs for !692 with rtph26xpay-aggregate-mode-default in 22 minutes and 53 seconds (queued for 4 seconds)
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gst-ios-13.2
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gst-macos-10.15
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  Integrate
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gst-ios-13.2
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documentation

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