Add caveat about expression syntax (#3362)

For if conditionals, expressions must be explicitly marked for evaluation if they contain any operators.

Resolves #3001

Co-authored-by: hubwriter <hubwriter@github.com>

Author: jidicula

data/reusables/github-actions/expression-syntax-if.md

@@ -1 +1 @@
-When you use expressions in an `if` conditional, you may omit the expression syntax ({% raw %}`${{ }}`{% endraw %}) because {% data variables.product.prodname_dotcom %} automatically evaluates the `if` conditional as an expression.
+When you use expressions in an `if` conditional, you may omit the expression syntax ({% raw %}`${{ }}`{% endraw %}) because {% data variables.product.prodname_dotcom %} automatically evaluates the `if` conditional as an expression, unless the expression contains any operators. If the expression contains any operators, the expression must be contained within {% raw %}`${{ }}`{% endraw %} to explicitly mark it for evaluation.