Followup tasks for nanvl

[sarutak]

Describe the bug

I added nanvl in #7171 but some followup tasks needed.
One is adding an entry to the function list table in user-guide/expressions.md for nanvl.
Another is modifying the error message in tests newly added to math_expressions.rs.

--- a/datafusion/physical-expr/src/math_expressions.rs
+++ b/datafusion/physical-expr/src/math_expressions.rs
@@ -1057,9 +1057,9 @@ mod tests {
             Arc::new(Float64Array::from(vec![5.0, 6.0, f64::NAN, f64::NAN])), // x
         ];
 
-        let result = nanvl(&args).expect("failed to initialize function atan2");
+        let result = nanvl(&args).expect("failed to initialize function nanvl");
         let floats =
-            as_float64_array(&result).expect("failed to initialize function atan2");
+            as_float64_array(&result).expect("failed to initialize function nanvl");
 
         assert_eq!(floats.len(), 4);
         assert_eq!(floats.value(0), 1.0);
@@ -1075,9 +1075,9 @@ mod tests {
             Arc::new(Float32Array::from(vec![5.0, 6.0, f32::NAN, f32::NAN])), // x
         ];
 
-        let result = nanvl(&args).expect("failed to initialize function atan2");
+        let result = nanvl(&args).expect("failed to initialize function nanvl");
         let floats =
-            as_float32_array(&result).expect("failed to initialize function atan2");
+            as_float32_array(&result).expect("failed to initialize function nanvl");

To Reproduce

No response

Expected behavior

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Additional context

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