doc/libav-merge: mention skipped outdev removal commit

Signed-off-by: James Almer <jamrial@gmail.com>

doc/libav-merge.txt

@@ -102,6 +102,7 @@ Stuff that didn't reach the codebase:
   - f44ec22e0 lavc: use av_cpu_max_align() instead of hardcoding alignment requirements
   - 4de220d2e frame: allow align=0 (meaning automatic) for av_frame_get_buffer()
 - Support recovery from an already present HLS playlist (see 16cb06bb30)
+- Remove all output devices (see 8e7e042d41 and https://ffmpeg.org/pipermail/ffmpeg-devel/2017-September/216904.html)
 
 Collateral damage that needs work locally:
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