Now that issue #12438 is resolved, this TODO can be completed.
Create a logf helper, which is similar to Server.logf method,
but takes a *Request to infer the *Server and its ErrorLog from.

Update documentation of Server.ErrorLog to mention a new type
of errors that may be logged to it.

Also update a statement in documentation of Server.ErrorLog from:

	// If nil, logging goes to os.Stderr via the log package's
	// standard logger.

To:

	// If nil, logging is done via the log package's standard logger.

The motivation for doing so is to avoid making inaccurate claims.
Logging may not go to os.Stderr if anyone overrides the log package's
default output via https://godoc.org/log#SetOutput. Saying that
the standard logger is used should be sufficient to explain the
behavior, and users can infer that os.Stderr is used by default,
unless it's changed.

Updates #12438.

Change-Id: I3a4b0db51d652fd25fb2065fbc2157a3dec4dd38
Reviewed-on: https://go-review.googlesource.com/53950Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>