[Docs] Update usage

docs/parts/usage.tex

@@ -31,7 +31,8 @@ Each of the options can also be overridden directly on a per-document
 
 
 \subsection{Letters}
-Letters require a different setup compared to a normal \LaTeX document:
+Letters require a different setup compared to a normal \LaTeX document.
+Be also aware that the usual commands featured by the helpers are \textit{not} available when using the letter helpers.
 
 \begin{minted}{latex}
 \documentclass[12pt,a4paper]{scrlttr2}