Add comment to parser.ExpressionSizeCodePointLimit (#435)

diff --git a/parser/options.go b/parser/options.go
index 436b427..3de76d0 100644
--- a/parser/options.go
+++ b/parser/options.go
@@ -50,6 +50,8 @@
 	}
 }
 
+// ExpressionSizeCodePointLimit is an option which limits the maximum code point count of an
+// expression.
 func ExpressionSizeCodePointLimit(expressionSizeCodePointLimit int) Option {
 	return func(opts *options) error {
 		if expressionSizeCodePointLimit < -1 {