updating node_apidoc to v0.12.7

[dyanarose]

Had a bit of a run around today as I had been reading the supplied v0.10.35 documentation for Buffer and hadn't quite grokked the Buffer._charsWritten comment, so I went into the repl to try it out.

However, turns out that _charsWritten has been removed from the Buffer class as of what looks like v0.11.x.

If it's appropriate to update the documentation, I can see about getting together a pull request.