[wp/doc] Document tactic: Validity

src/plugins/wp/doc/manual/wp_plugin.tex

@@ -378,6 +378,12 @@ Finally, when the user select a arithmetic comparison over $a$ and $b$, the tact
 \Delta,a>b&\models G
 \end{array}} \]
 
+\paragraph{Validity} Unfold validity and range definitions\\
+The user selects a validity expression (\lstinline{valid_rd},
+\lstinline{valid_rw}, \lstinline{invalid} or \lstinline{included}).
+The expression is unfolded to a \textsf{Qed} term.
+
+
 \subsubsection{Over integers}
 
 \paragraph{BitRange} Range of logical bitwise operators \\