I believe those are just basic vector operations. scaleBy appears to be simple scalar multiplication, and truncate appears to mean "if the vector's magnitude > x then scale it so it equals x".
A quick example:
Let v be the 2D vector (3,4).
The magnitude of v is sqrt(3^2 + 4^2) = 5.
If we scale v by a factor of 2, we get 2 * (3,4) = (2*3, 2*4) = (6, 4), which has a magnitude of sqrt(6^2 + 4^2) = 10. Notice that's 2 times the original magnitude.
If we truncate v to have a magnitude of at most 3, that would mean multiplying v by the scalar 3/5, which gets us (1.8, 2.4).
I'd recommend googling some tutorials on vector arithmetic in general since you'll want to know a lot more than just this if you intend on implementing anything resembling physics or intelligent movement.
To answer your other question, since these operations are so simple (once you understand them), SFML and the C++ libraries don't bother providing them directly. The closest thing I know of is a couple functions in the standard <complex> header. For instance, std::norm calculates the norm of a complex number, which is basically the same thing as the magnitude of a 2D vector (since a complex number is typically represented with a 2D vector). But even then, it might be better to implement these functions yourself if only to ensure you actually understand them.
P.S. This has absolutely nothing to do with functional programming, so don't use that term.