ArrayBufferWriter<T>.ResetWrittenCount has no summary comment

[amcasey]

There's no description of the member on its own page or the page for the containing type. It looks like there's a comment in the source code though, so I'm not sure why it wasn't picked up automatically.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[adamsitnik]

cc @carlossanlop