Yes you are: specifically, your job as a manager is to make the company and your people successful. The way that is achieved is through collaboration.
If somebody is asking questions to a disruptive extent, then you reach out to them or their manager to have an honest conversation about it, and to level expectations about what requests for help are unreasonable.
> The question is whether one employee's time is more valuable than another employee's time
That is looking at the world as a zero-sum game. If you devote 10 minutes to help somebody, and they're 10 minutes more efficient at that task every time afterwards, then the company wins. It's not your time, it's company time, and most companies prefer that their time be spent on the most value-added thing, which is almost always force multiplication
The question is whether one employee's time is more valuable than another employee's time. One employee answering another employee's question expends two people's time. If the answer is in the book or on the first page of google, it is wasting one of those employee's time, and that is the boss's money. If the employee asking the question makes ten times as much as the employee answering the question, and it saves him 10% of his time over flipping through the manual, it's a wash. If both employees make the same amount, or the other person even makes less, it is obviously a waste.
This is simple.
RTFM isn't an angsty insult. It is a suggestion because I am not going to answer your question if it is in the manual.