OpenOptions::append documentation should clarify file creation behavior

[mqudsi]

The documentation for OpenOptions is somewhat incomplete as it confusingly leaves out any mention of the file creation behavior.

The current description reads

This option, when true, means that writes will append to a file instead of overwriting previous contents. Note that setting .write(true).append(true) has the same effect as setting only .append(true).

While it is true that append() only sets (or clears) the append flag, I think it might be wise to include something to the effect of

If the file does not already exist at the specified path, .create(true) must also be supplied prior to calling .open() or else an error will be raised.