flag set error : log to file or stderr

[THMAIL]

/kind bug

What steps did you take and what happened:
I try to save log to local file with parameter ：go run main.go -log_dir=logs -logtostderr=false
But errorLogs still will print to stderr!(infoLogs won't)
As the doc say "stderrthreshold : logs at or above this threshold go to stderr when writing to files and stderr (no effect when -logtostderr=true or -alsologtostderr=false.
So I tried go run main.go -log_dir=logs -logtostderr=false -alsologtostderr=true -stderrthreshold=2 and go run main.go -log_dir=logs -logtostderr=false -alsologtostderr=true -stderrthreshold=3, but it doesn't work at all.
The errorLogs still print to stderr.
So I analyzed the code of klog， finally I find that ：

if alsoToStderr || l.alsoToStderr || s >= l.stderrThreshold.get() {
	os.Stderr.Write(data)
}

in klog.go file Line 20.
this code tell me I should set alsoToStderr to false, then the stderrThreshold can come into effect. This is contrary to what the documentation says！
I tried go run main.go -log_dir=logs -logtostderr=false -alsologtostderr=false -stderrthreshold=3. It worked ! the errorLog didn't print to stderr.

What did you expect to happen:
please fix this bug or just update the doc, it's up to you.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

[pohly]

The code is the source of truth for this, because users might rely on this behavior.

Can you propose a PR which fixes the docs?

[logicalhan]

/triage accepted
/assign @thockin