This is the correct documentation:
ffmpeg version 4.1.git Copyright (c) 2000-2019 the FFmpeg developers
built with Microsoft (R) C/C++-Optimierungscompiler Version 19.15.26732.1 f├╝r x64
configuration: --toolchain=msvc --extra-cflags='-MDd -I/c/TeamCity/buildAgent/work/ffmpeg_nightly_debug/build/include' --extra-ldflags='-LIBPATH:/c/TeamCity/buildAgent/work/ffmpeg_nightly_debug/build/lib' --prefix=/c/TeamCity/buildAgent/work/ffmpeg_nightly_debug/build --pkg-config-flags=--static --disable-doc --disable-shared --enable-static --enable-runtime-cpudetect --disable-devices --disable-network --enable-w32threads --enable-gpl --enable-libmp3lame --enable-libzimg --enable-avisynth --enable-libx264 --enable-libx265 --enable-cuda --enable-cuvid --enable-d3d11va --enable-nvenc --enable-amf --enable-libfdk-aac --enable-libvpx --enable-libvorbis --enable-libopus
libavutil 56. 26.100 / 56. 26.100
libavcodec 58. 52.100 / 58. 52.100
libavformat 58. 27.103 / 58. 27.103
libavdevice 58. 7.100 / 58. 7.100
libavfilter 7. 49.100 / 7. 49.100
libswscale 5. 4.100 / 5. 4.100
libswresample 3. 4.100 / 3. 4.100
libpostproc 55. 4.100 / 55. 4.100
Encoder prores_ks [Apple ProRes (iCodec Pro)]:
General capabilities: threads intraonly
Threading capabilities: frame and slice
Supported pixel formats: yuv422p10le yuv444p10le yuva444p10le
ProRes encoder AVOptions:
-mbs_per_slice <int> E..V..... macroblocks per slice (from 1 to 8) (default 8)
-profile <int> E..V..... (from -1 to 5) (default auto)
auto E..V.....
proxy E..V.....
lt E..V.....
standard E..V.....
hq E..V.....
4444 E..V.....
4444xq E..V.....
-vendor <string> E..V..... vendor ID (default "Lavc")
-bits_per_mb <int> E..V..... desired bits per macroblock (from 0 to 8192) (default 0)
-quant_mat <int> E..V..... quantiser matrix (from -1 to 6) (default auto)
auto E..V.....
proxy E..V.....
lt E..V.....
standard E..V.....
hq E..V.....
default E..V.....
-alpha_bits <int> E..V..... bits for alpha plane (from 0 to 16) (default 16)I just tested it. 1 bit actually works.
