docs: Clarify how to log both to a custom transport and to stdout

[kibertoad]

As evidenced by #1741, and also by myself failing to understand how to do this despite trying to read documentation, this (fairly common) use-case might benefit from a more explicit section.

Yes, it is also mentioned in documentation for destination of "pino/file", but I've skipped the whole section because it's called "file" and its description says "The pino/file transport routes logs to a file (or file descriptor).". Not everyone knows that stdout is a file descriptor - maybe we should make that part more clear as well, e. g. saying "(or file descriptor, such as 1 for stdout)".