manual: add warning about floating-point arithmetic #1050

Per a suggestion from @muzimuzhi on #1050, add a warning about floating-point arithmetic
to the first introduction of `foreach` in Karl's tutorial.

doc/generic/pgf/text-en/pgfmanual-en-tutorial.tex

@@ -1100,11 +1100,16 @@ \subsection{Repeating Things: For-Loops}
 \end{codeexample}
 
 If you provide \emph{two} numbers before the |...|, the |\foreach| statement
-will use their difference for the stepping:
+will use their difference for the stepping, but be wary of floating-point
+rounding errors:
 %
 \begin{codeexample}[]
-\tikz \foreach \x in {-1,-0.5,...,1}
-       \draw (\x cm,-1pt) -- (\x cm,1pt);
+\begin{tikzpicture}
+  \foreach \x in {-1,-0.5,...,1}
+    \draw (\x cm,-1pt) -- (\x cm,1pt);
+  \foreach \x in {-1,-0.8,...,1}
+    \draw (\x cm,-7pt) -- (\x cm,-5pt);
+\end{tikzpicture}
 \end{codeexample}
 
 We can also nest loops to create interesting effects: