Sorry for not submitting a pull request to cover this.

The camera.md documentation does not mention the flicker avoidance flag that was merged into raspivid in July 2017 (see PR 406).

This adds the flicker avoidance flag --flicker or -fli that takes four options off, auto, 50Hz and 60Hz (the default is off).

I don't know how one determines whether 50Hz or 60Hz is appropriate for one's particular situation but I found setting -fli auto dramatically reduced the flicker I saw when using the Raspberry Pi camera module V1 (I haven't tried it with the V2 module) to stream from my Pi to my laptop.

So given this improvement, it definitely seems worth documenting for others. It would also be useful to know if there are any downsides to enabling flicker avoidance (otherwise why isn't auto the default rather than off). E.g. maybe it has CPU usage consequences or affects things like the maximum FPS.