message: docs: fix description of flow control
  Risk Level: Low
  Docs Changes: This pull request fixes typos and the descriptions about calling readDisable on high/low watermarks.
diff: @@ -135,7 +135,7 @@ time, it should return `FilterDataStatus::StopIterationAndWatermark` to pause
  further data processing, which will cause the `ConnectionManagerImpl` to trigger
  watermark callbacks on behalf of the filter. If a filter can not make forward progress without the
  complete body, it should return `FilterDataStatus::StopIterationAndBuffer`.
  -in this case if the `ConnectionManagerImpl` buffers more than the allowed data
  +In this case if the `ConnectionManagerImpl` buffers more than the allowed data
  it will return an error downstream: a 413 on the request path, 500 or `resetStream()` on the
  response path.
  @@ -165,7 +165,7 @@ And the low watermark path:
  `StreamDecoderFilterCallback::onDecoderFilterBelowWriteBufferLowWatermark()`.
  * When `Envoy::Http::ConnectionManagerImpl` receives
  `onDecoderFilterAboveWriteBufferHighWatermark()` it calls `readDisable(false)` on the downstream
  - stream to pause data.
  + stream to resume data.
  # Encoder filters
  @@ -192,11 +192,11 @@ The encoder high watermark path for streaming filters is as follows:
  `DownstreamWatermarkCallbacks::onAboveWriteBufferHighWatermark()` for all
  filters which registered to receive watermark events
  * `Envoy::Router::Filter` receives `onAboveWriteBufferHighWatermark()` and calls
  - `readDisable(false)` on the upstream request.
  + `readDisable(true)` on the upstream request.
  The encoder low watermark path for streaming filters is as follows:
  - * When an instance of `Envoy::Router::StreamEncoderFilter` buffers too much data it should call
  + * When an instance of `Envoy::Router::StreamEncoderFilter` buffers drains it should call
  `StreamEncoderFilterCallback::onEncodeFilterBelowWriteBufferLowWatermark()`.
  * When `Envoy::Http::ConnectionManagerImpl::ActiveStreamEncoderFilter` receives
  `onEncoderFilterBelowWriteBufferLowWatermark()` it calls
  @@ -205,7 +205,7 @@ The encoder low watermark path for streaming filters is as follows:
  `DownstreamWatermarkCallbacks::onBelowWriteBufferLowWatermark()` for all
  filters which registered to receive watermark events
  * `Envoy::Router::Filter` receives `onBelowWriteBufferLowWatermark()` and calls
  - `readDisable(true)` on the upstream request.
  + `readDisable(false)` on the upstream request.
  # HTTP and HTTP/2 codec upstream send buffer
  @@ -394,6 +394,8 @@ watermark path is as follows:
  From this point the `ConnectionManagerImpl` takes over and the code path is the same as for the
  HTTP/2 codec downstream send buffer.
  +The low watermark path is as follows:
  +
  * When `Http::Http1::ConnectionImpl::output_buffer_` drains
  it calls `onOutputBufferBelowLowWatermark()`
  * Http::Http1::ConnectionImpl::ServerConnectionImpl::onOutputBufferBelowLowWatermark() calls
  @@ -416,6 +418,8 @@ watermark path is as follows:
  From this point on the `Envoy::Router::Filter` picks up the event and the code path is the same as
  for the HTTP/2 codec upstream send buffer.
  +The low watermark path is as follows:
  +
  * When `Http::Http1::ConnectionImpl::output_buffer_` drains
  it calls `onOutputBufferBelowLowWatermark()`
  * Http::Http1::ConnectionImpl::ClientConnectionImpl::onOutputBufferBelowLowWatermark() calls
  