message: Update documentation on VideoConfig.simulcast_config.
  Cr-Commit-Position: refs/heads/master@{#28703}
diff: @@ -176,9 +176,11 @@ class PeerConnectionE2EQualityTestFixture {
  // If presented video will be transfered in simulcast/SVC mode depending on
  // which encoder is used.
  //
  - // Simulcast is supported only from 1st added peer and for now only for
  - // Vp8 encoder. Also RTX doesn't supported with simulcast and will
  - // automatically disabled for tracks with simulcast.
  + // Simulcast is supported only from 1st added peer. For VP8 simulcast only
  + // without RTX is supported so it will be automatically disabled for all
  + // simulcast tracks. For VP9 simulcast enables VP9 SVC mode and support RTX,
  + // but only on non-lossy networks. See more in documentation to
  + // VideoSimulcastConfig.
  absl::optional<VideoSimulcastConfig> simulcast_config;
  // If specified the input stream will be also copied to specified file.
  // It is actually one of the test's output file, which contains copy of what
  