Fix #408 and improve chart description (#411)

This improves the figure's description.

book/sourdough-starter/sourdough-starter.tex

@@ -283,9 +283,12 @@ starters.
 \centering
   \input{figures/fig-starter-readiness.tex}
   \caption[Determining sourdough starter readiness]{A flow chart showing you how to
-      determine if your sourdough starter is ready to be used.  For checking
-      readiness look at a size increase and take note of your starter's smell.
-      Both are important indicators to check for readiness.}%
+      determine if your sourdough starter is ready to be used. Make sure to
+      wait at least \qtyrange{6}{12}{\hour} after feeding your
+      starter to check its readiness. To evaluate it, look at your starter's size
+      increase, airy texture and take note of its smell.
+      All three factors are important to properly evaluate your starter's activity level.
+      An active starter is an important foundation for a successful dough fermentation}%
   \label{fig:sourdough-starter-readiness}
 \end{flowchart}