doc/libav-merge: mention skipped or incomplete runtime alignment commits

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

doc/libav-merge.txt

@@ -98,6 +98,9 @@ Stuff that didn't reach the codebase:
 - Removal of the custom atomic API (5cc0057f49, see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-March/209003.html)
 - new bitstream reader (see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-April/209609.html)
 - use of the bsf instead of our parser for vp9 superframes (see fa1749dd34)
+- use av_cpu_max_align() instead of hardcoding alignment requirements (see https://ffmpeg.org/pipermail/ffmpeg-devel/2017-September/215834.html)
+  - f44ec22e0 lavc: use av_cpu_max_align() instead of hardcoding alignment requirements
+  - 4de220d2e frame: allow align=0 (meaning automatic) for av_frame_get_buffer()
 
 Collateral damage that needs work locally:
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