clarify relation to literature

Doc/UserManual/FFCompute.tex

@@ -120,8 +120,9 @@ to deal with the second case in~(\ref{Eexpcancel}).
 
 To derive the form factor of prisms and pyramids with polygonal bases
 we need the form factor of an arbitrary planar polygon~$\Pi$.
-The solution is known from the literature~\cite{LeMi83},
-but shall be derived here by a different method.
+The solution avalaible from the literature~\cite{LeMi83}
+contains singularities and does not provide an obvious way how handle them.
+Here, a different expression shall be derived by a different method.
 The plane of the polygon has a normal~$\n$,
 which induces a decomposition of any vector~$\x$ into normal
 and in-plane components