content(warnings): Add W6-W7, X2-X3, N4 — energy misconceptions + cross-refs

concepts/mechanics/u3/m3-1-work.tex

@@ -19,6 +19,8 @@ Therefore the \emph{total work} done by the force as the particle moves along th
 W=\int_C \vec{F}\cdot d\vec{r}=\int_C F_{\parallel}\,ds.
 \]}
 
+This definition of work is the foundation for the work-energy theorem (Section 3.2) and the potential-energy formalism (Section 3.3).
+
 \thm{Line-integral form of work and the constant-force special case}{Let a particle move from an initial point to a final point along a path $C$ under a force $\vec{F}$. Then the work done by that force is
 \[
 W=\int_C \vec{F}\cdot d\vec{r}.