yocto: slides: improve "Introduction to recipe extensions" slide

Make wording more precise and also mention removing from variables.

Signed-off-by: Luca Ceresoli <[email protected]>

slides/yocto-recipe-advanced/yocto-recipe-advanced.tex

@@ -18,10 +18,16 @@ \subsection{Extending a recipe}
 \begin{frame}
   \frametitle{Introduction to recipe extensions}
   \begin{itemize}
-    \item Metadata can be changed, added or appended.
-    \item Tasks can be added or appended.
-    \item Operators are used extensively, to add, append, prepend or
-          assign values.
+    \item Variable values can be modified
+      \begin{itemize}
+      \item Using operators to set, append or prepend
+      \item Using overrides to append, prepend or remove
+      \end{itemize}
+    \item Tasks can be modified
+      \begin{itemize}
+      \item Using overrides to append or prepend task code
+      \item Adding new tasks
+      \end{itemize}
   \end{itemize}
 \end{frame}