It's probably not stated explicitly, but implied here (emphasis mine):
Additionally, the return value of this function is fmt::Result which is a type alias of Result<(), std::fmt::Error>. Formatting implementations should ensure that they propagate errors from the Formatter (e.g., when calling write!). However, they should never return errors spuriously. That is, a formatting implementation must and may only return an error if the passed-in Formatter returns an error. This is because, contrary to what the function signature might suggest, string formatting is an infallible operation. This function only returns a result because writing to the underlying stream might fail and it must provide a way to propagate the fact that an error has occurred back up the stack.