Adding helpful note about legendCallback (chartjs#5094)

I didn't realize you had to call generateLegend() manually and a quick search tells me that lots of other people have run into this too.

docs/configuration/legend.md

@@ -164,3 +164,7 @@ var chart = new Chart(ctx, {
     }
 });
 ```
+
+Note that legendCallback is not called automatically and you must call `generateLegend()` yourself in code when creating a legend using this method.
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+