content(warnings): Add W8-W11, X4-X5 — collision, inertia, and torque cross-refs

concepts/mechanics/u5/m5-3-torque.tex

@@ -30,6 +30,8 @@ For rotation about a chosen fixed axis with unit vector $\hat{k}$,
 \]
 so the scalar $\tau$ is positive or negative according to the declared sign convention.}
 
+Torque about a fixed axis connects directly to angular acceleration through Newton's second law for rotation (Section 5.6), just as net force connects to linear acceleration through Newton's second law (Section 2.1).
+
 \pf{Why $rF\sin\phi$ equals $F\ell$}{From the cross-product magnitude formula,
 \[
 |\vec{\tau}|=|\vec{r}\times \vec{F}|=rF\sin\phi.