Add links to docs

Author: nuno-faria

datafusion/sql/src/unparser/dialect.rs

@@ -620,6 +620,7 @@ impl Dialect for MySqlDialect {
             return date_part_to_sql(unparser, self.date_field_extract_style(), args);
         }
 
+        // https://dev.mysql.com/doc/refman/9.7/en/mathematical-functions.html#function_rand
         if func_name == "random" {
             return Ok(Some(unparser.scalar_function_to_sql("rand", args)?));
         }
@@ -764,6 +765,7 @@ impl Dialect for BigQueryDialect {
             return date_part_to_sql(unparser, self.date_field_extract_style(), args);
         }
 
+        // https://docs.cloud.google.com/bigquery/docs/reference/standard-sql/mathematical_functions#rand
         if func_name == "random" {
             return Ok(Some(unparser.scalar_function_to_sql("rand", args)?));
         }