added hints which libraries need to be loaded as well to make the exa…

…mple in `pgfmanual-en-tikz-graphs.tex` work closes issue #755)

doc/generic/pgf/text-en/pgfmanual-en-tikz-graphs.tex

@@ -243,8 +243,9 @@ \subsubsection{Concept: Chain Groups}
 
 As can be seen, the placement is not particularly nice by default, use the
 algorithms from the graph drawing libraries to get a better layout. For
-instance, adding |tree layout| to the above code results in the following
-somewhat more pleasing rendering:
+instance, adding |tree layout| to the above code (and
+|\usetikzlibrary{graphdrawing}| as well as |\usegdlibrary{trees}| to the
+preamble) results in the following somewhat more pleasing rendering:
 %
 \ifluatex
 \medskip