Fix an out of date comment for snapshot_physical_expr (#18498)

## Which issue does this PR close?


- Closes #18497

## Rationale for this change
Better docs

## What changes are included in this PR?

Docs clarifications

## Are these changes tested?

N/A

## Are there any user-facing changes?

No

Author: AdamGS

datafusion/physical-expr-common/src/physical_expr.rs

@@ -565,9 +565,8 @@ pub fn fmt_sql(expr: &dyn PhysicalExpr) -> impl Display + '_ {
 ///
 /// # Returns
 ///
-/// Returns an `Option<Arc<dyn PhysicalExpr>>` which is the snapshot of the
-/// `PhysicalExpr` if it is dynamic. If the `PhysicalExpr` does not have
-/// any dynamic references or state, it returns `None`.
+/// Returns a snapshot of the `PhysicalExpr` if it is dynamic, otherwise
+/// returns itself.
 pub fn snapshot_physical_expr(
     expr: Arc<dyn PhysicalExpr>,
 ) -> Result<Arc<dyn PhysicalExpr>> {