Improve documentation for the new video encoder API.

This CL adds mostly automated formatting and style fixes. One functional
change is this added paragraph under the section for `num_buffers` of
the prediction constraints:
```
  If the encoder implementation
  supports reference frame control, it must indicate at least a value of 1.
  Implementations that don't support reference frame control must indicate a
  value of 0.
```

Bug: webrtc:496266459
Change-Id: Ib9dd658ec4635e4da1e081e84b56260be7cb7ab5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/497302
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48415}