Attribute Macro user_doc

#[user_doc]

This procedural macro is intended to parse a rust custom attribute and create user documentation from it by constructing a DocumentBuilder() automatically. The Documentation can be retrieved from the documentation() method declared on AggregateUDF, WindowUDFImpl, ScalarUDFImpl traits. For doc_section, this macro will try to find corresponding predefined DocSection by label field Predefined DocSection can be found in datafusion/expr/src/udf.rs Example:

#[user_doc(
    doc_section(label = "Time and Date Functions"),
    description = r"Converts a value to a date (`YYYY-MM-DD`).",
    syntax_example = "to_date('2017-05-31', '%Y-%m-%d')",
    sql_example = r#"```sql
> select to_date('2023-01-31');
+-----------------------------+
| to_date(Utf8(\"2023-01-31\")) |
+-----------------------------+
| 2023-01-31                  |
+-----------------------------+
```"#,
    standard_argument(name = "expression", prefix = "String"),
    argument(
        name = "format_n",
        description = r"Optional [Chrono format](https://docs.rs/chrono/latest/chrono/format/strftime/index.html) strings to use to parse the expression. Formats will be tried in the order
  they appear with the first successful one being returned. If none of the formats successfully parse the expression
  an error will be returned."
   )
)]
#[derive(Debug)]
pub struct ToDateFunc {
    signature: Signature,
}

will generate the following code

pub struct ToDateFunc {
    signature: Signature,
}
impl ToDateFunc {
    fn doc(&self) -> Option<&datafusion_doc::Documentation> {
        static DOCUMENTATION: std::sync::LazyLock<
            datafusion_doc::Documentation,
        > = std::sync::LazyLock::new(|| {
            datafusion_doc::Documentation::builder(
                    datafusion_doc::DocSection {
                        include: true,
                        label: "Time and Date Functions",
                        description: None,
                    },
                    r"Converts a value to a date (`YYYY-MM-DD`).".to_string(),
                    "to_date('2017-05-31', '%Y-%m-%d')".to_string(),
                )
                .with_sql_example(
                    r#"```sql
> select to_date('2023-01-31');
+-----------------------------+
| to_date(Utf8(\"2023-01-31\")) |
+-----------------------------+
| 2023-01-31                  |
+-----------------------------+
```"#,
                )
                .with_standard_argument("expression", "String".into())
                .with_argument(
                    "format_n",
                    r"Optional [Chrono format](https://docs.rs/chrono/latest/chrono/format/strftime/index.html) strings to use to parse the expression. Formats will be tried in the order
they appear with the first successful one being returned. If none of the formats successfully parse the expression
an error will be returned.",
                )
                .build()
        });
        Some(&DOCUMENTATION)
    }
}