ArrayBufferWriter<T>.ResetWrittenCount has no summary comment

There's no description of the member on [its own page](https://learn.microsoft.com/en-us/dotnet/api/system.buffers.arraybufferwriter-1.resetwrittencount?view=net-8.0#system-buffers-arraybufferwriter-1-resetwrittencount) or the page for the [containing type](https://learn.microsoft.com/en-us/dotnet/api/system.buffers.arraybufferwriter-1?view=net-8.0). It looks like there's a comment in the [source code](https://github.com/dotnet/runtime/blob/75dc9135818c14cafc8195b0b3b23f96f6e708f2/src/libraries/Common/src/System/Buffers/ArrayBufferWriter.cs#L112) though, so I'm not sure why it wasn't picked up automatically.