fix: prevent pie chart crash on zero or negative values

docs/syntax/pie.md

@@ -37,6 +37,11 @@ Drawing a pie chart is really simple in mermaid.
   - Followed by `:` colon as separator
   - Followed by `positive numeric value` (supported up to two decimal places)
 
+**Note:**
+
+> Pie chart values must be **positive numbers greater than zero**.\
+> **Zero and negative values are not allowed** and will result in an error.
+
 \[pie] \[showData] (OPTIONAL)
 \[title] \[titlevalue] (OPTIONAL)
 "\[datakey1]" : \[dataValue1]