[EN13445-3 2021 - 16.5] Allowable bounded area with 0 shear stress

Add more explanations concerning the allowable bounded area with 0 shear stress graphs when designing a nozzle on a cylindrical/spherical shell (e.g showing that it is the eq 16.5-25 taken with different values of Mx and My at design pressure).

Also editing graphs resolving inequation 16.5-15 the same way would give a accurate view of the allowable loads and moments.