docs: update fill_null documentation with detailed usage examples

Author: kosiew

docs/source/library-user-guide/using-the-dataframe-api.md

@@ -198,17 +198,19 @@ async fn main() -> Result<()> {
 }
 ```
 
-[`custom_file_format.rs`]: https://github.com/apache/datafusion/blob/main/datafusion-examples/examples/custom_file_format.rs
+## Fill Null Values with DataFrame::fill_null
 
-The output file will look like (Example Output):
+The `fill_null` method allows you to replace null values in specified columns with a provided value. When an empty list of columns is passed, the operation is applied to all columns. The method only fills if the given value can be cast to the column's data type.
 
-```sql
-> select * from '../datafusion/core/example.parquet';
-+---+---+---+
-| a | b | c |
-+---+---+---+
-| 1 | 2 | 3 |
-+---+---+---+
+**Usage Example:**
+
+```rust
+// Fill nulls in specific columns "a" and "c":
+let df = ctx.read_csv("tests/data/example.csv", CsvReadOptions::new()).await?;
+let df = df.fill_null(ScalarValue::from(0), vec!["a".to_owned(), "c".to_owned()])?;
+
+// Fill nulls across all columns:
+let df = df.fill_null(ScalarValue::from(0), vec![])?;
 ```
 
 ## Relationship between `LogicalPlan`s and `DataFrame`s