Add performance health warning. Remove Beanshell preference advice as…

… it is not relevant here, except specifically the bit on leading zeros. Generic advice on Beanshell vs Legacy is given in the general section on expressions.

vassal-doc/src/main/readme-referencemanual/ReferenceManual/Label.adoc

@@ -22,9 +22,12 @@ The text of the label can be fixed or specifiable by a player at game time.
 . Check if the entered label is a $$ Variable Substitution (defined by a string enclosed between $ characters). This allows <<Properties.adoc#top,Properties>> of the piece to be substituted with $..$ notation, as in a <<MessageFormat.adoc#top,Message Format>>
 . If no check is true the entered string is displayed.
 
+****
+**Note!** _Label Text_ will be reevaluated every time a piece bearing the Text Label trait is  drawn, which will occur frequently in a typical module whenever such a piece is displayed. Therefore, longer, more convoluted Beanshell expressions (e.g., comparisons, arithmetic, Count/Sums etc.) may affect performance, especially with widespread use.
+****
+
 A known limitation of the evaluation order above is that if you enter a sequence of digits starting with 0s, these will be removed when the string is evaluated as integer.
 A workaround to this is to enter the same string via a Beanshell expression. For example to display a sequence of 3 zeroes as label text you can use *{"000"}*.
-Note that the Beanshell expressions should be preferred to $$ Variable Substitutions. The latter are only available for historical reasons.
 
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