Improved literature footnote.

Doc/UserManual/Scattering.tex

@@ -590,15 +590,20 @@ which implies that $\psi_\si$ and $\psi_\sf$ are plane waves.
 In the distorted-wave Born approximation (DWBA),\footnote{
 The DWBA was originally devised by Massey and Mott (ca 1933)
 for collisions of charged particles.
+Summaries can be found in some quantum mechanics textbooks (Messiah, Schiff)
+and in monographs on scattering theory (e.~g.\ Newton).
 The first explicit applications to grazing-incidence scattering
-were published 1982:
+were published in 1982:
 Vineyard \cite{Vin82} discussed X-ray scattering,
 but failed to account for the distortion of the scattered wave;
-Mazur and Mills \cite{MaMi82} derived the inelastic neutron scattering cross section
+Mazur and Mills \cite{MaMi82} deployed heavy formalism
+to compute the inelastic neutron scattering cross section
 of ferromagnetic surface spin waves from scratch.
 A concise derivation of the DWBA cross section
-was provided by Dietrich and Wagner (1984/85) for X-rays \cite{DiWa84}
-and neutrons \cite{DiWa85}.}
+was provided by Dietrich and Wagner (1984/85)
+for X-rays \cite{DiWa84} and neutrons \cite{DiWa85}.
+Unfortunately, their work was overlooked in much of the later literature,
+which often fell back to less convincing derivations.}
 this requirement is dropped.
 The SLD decomposition~\cref{Edecompose_v}
 is restored to full genericity,