docs: add missing documentation about copyfrom (#3583)

Author: YvanDaSilva

docs/reference/config.md

@@ -135,6 +135,8 @@ The `gen` mapping supports the following keys:
   - Output directory for generated code.
 - `sql_package`:
   - Either `pgx/v4`, `pgx/v5` or `database/sql`. Defaults to `database/sql`.
+- `sql_driver`:
+  - Either `github.com/jackc/pgx/v4`, `github.com/jackc/pgx/v5`, `github.com/lib/pq` or `github.com/go-sql-driver/mysql`. No defaults. Required if query annotation `:copyfrom` is used.
 - `emit_db_tags`:
   - If true, add DB tags to generated structs. Defaults to `false`.
 - `emit_prepared_queries`:

docs/reference/query-annotations.md

@@ -222,3 +222,9 @@ func (b *CreateBookBatchResults) Close() error {
 	//...
 }
 ```
+
+## `:copyfrom`
+
+__NOTE: This command is driver and package specific, see [how to insert](../howto/insert.md#using-copyfrom)
+
+This command is used to insert rows a lot faster than sequential inserts.