Add in an example of how to use separated (#1902)

* Add in an example of how to use separated

Dearest Maintainer,

Thank you for your work on this project. I started using query builder today and I have enjoyed it. I did have a hard time figuring out how best to use separated to generate the values for an IN statement. It is my hope that adding an example will save someone else a few minutes of code reading or compile time. I wrote the example in the github text editor but It looks correct.    

Thanks again for your work on this.
Becker

* end ```

* Apply cfg and end ```

* remove dup

* Update sqlx-core/src/query_builder.rs

sqlx-core/src/query_builder.rs

@@ -139,6 +139,28 @@ where
     /// [`.push_bind()`][Separated::push_bind] methods which push `separator` to the query
     /// before their normal behavior. [`.push_unseparated()`][Separated::push_unseparated] is also
     /// provided to push a SQL fragment without the separator.
+    ///
+    /// ```rust
+    /// # #[cfg(feature = "mysql")] {
+    /// use sqlx::{Execute, MySql, QueryBuilder};
+    /// let foods = vec!["pizza".to_string(), "chips".to_string()];
+    /// let mut query_builder: QueryBuilder<MySql> = QueryBuilder::new(
+    ///     "SELECT * from food where name in ("
+    /// );
+    /// // One element vector is handled correctly but an empty vector
+    /// // would cause a sql syntax error
+    /// let mut separated = query_builder.separated(", ");
+    /// for value_type in foods.iter() {
+    ///   separated.push_bind(value_type);
+    /// }
+    /// separated.push_unseparated(") ");
+    ///
+    /// let mut query = query_builder.build();
+    /// let sql = query.sql();
+    /// assert!(sql.ends_with("in (?, ?) "));
+    /// # }
+    /// ```
+
     pub fn separated<'qb, Sep>(&'qb mut self, separator: Sep) -> Separated<'qb, 'args, DB, Sep>
     where
         'args: 'qb,
@@ -249,6 +271,7 @@ where
     /// // 16383 * 4 = 65532
     /// assert_eq!(arguments.len(), 65532);
     /// # }
+    /// ```
     pub fn push_values<I, F>(&mut self, tuples: I, mut push_tuple: F) -> &mut Self
     where
         I: IntoIterator,