[Eva] User manual: adds paragraph about the alternative syntax of Eva directives.

doc/eva/main.tex

@@ -5440,6 +5440,14 @@ This section lists such directives supported by \Eva{}, with a short description
 of their behavior and a reference to the relevant section of this manual that
 explains in more details their effect on the analysis.
 
+Each of these directives also has an alternative syntax where the keyword is
+prefixed by \texttt{\textbackslash eva::}. Thus, one can write
+\texttt{//@ \textbackslash eva::split x; //@ loop \textbackslash eva::unroll 100;}
+instead of \texttt{//@ split x; //@ loop unroll 100;}.
+This syntax makes explicit that the directive is specific to the \Eva{} plug-in,
+and avoids parsing failure if \Eva{} is not loaded (for instance when using
+the \verb|-no-autoload-plugins| parameter).
+
 \subsection{Directives guiding trace partitioning}
 
 \begin{center}