Where does the relation $\theta =(\theta_\mathrm{sf}^2-\theta_\mathrm{dif}^2)^{1/2}$, for combining self-focusing and diffraction effects, come from?

I have some troubles understanding one statement in the book "Nonlinear Optics" by R.Boyd. In chapter 7.1, it is derived that in a nonlinear material a light beam undergoes self-focusing due to the dependence of the refractive index on the intensity of the beam itself, and the value of the corresponding self-focusing angle is obtained.

A few lines below, it is stated that, in the case of small laser power, the convergence angle is reduced by diffraction; the total angle is said to be given by $$\begin{equation} \theta =(\theta_\mathrm{sf}^2-\theta_\mathrm{dif}^2)^{1/2} \end{equation}$$

I can't get to understand why this equality holds.. can someone help me? In the figure below you can see the original paragraph from the book (equation 7.1.4 is just the expression for $$\theta_\mathrm{sf}$$, which I think is unnecessary)

The derivation to the result given by Eq. (7.1.4) ignores the effects of diffraction, and thus might be expected to be valid when self-action effects overwhelm those of diffraction$$-$$that is, for $$P\gg P_\mathrm{cr}$$. For smaller laser powers, the self-focusing distance can be estimated by noting that the beam convergence angle is reduced by diffraction effects and is given approximately by $$\theta =(\theta_\mathrm{sf}^2-\theta_\mathrm{dif}^2)^{1/2}$$, where $$\theta_\mathrm{dif} = 0.61\lambda_0/n_0d \tag{7.1.5}$$ is the diffraction angle of a beam of diameter $$d$$ and vacuum wavelength $$\lambda_0$$.