docs: add example for postgres enums with type TEXT (#3655)

* docs: add example for postgres enums with type TEXT

Signed-off-by: tison <wander4096@gmail.com>

* revert GitHub naming

Signed-off-by: tison <wander4096@gmail.com>

* add note

Signed-off-by: tison <wander4096@gmail.com>

---------

Signed-off-by: tison <wander4096@gmail.com>

Author: tisonkun

sqlx-postgres/src/types/mod.rs

@@ -179,6 +179,18 @@
 //! enum Mood { Sad = 0, Ok = 1, Happy = 2 }
 //! ```
 //!
+//! Rust enumerations may also be defined to be represented as a string using `type_name = "text"`.
+//! The following type expects a SQL type of `TEXT` and will convert to/from the Rust enumeration.
+//!
+//! ```rust,ignore
+//! #[derive(sqlx::Type)]
+//! #[sqlx(type_name = "text")]
+//! enum Mood { Sad, Ok, Happy }
+//! ```
+//!
+//! Note that an error can occur if you attempt to decode a value not contained within the enum
+//! definition.
+//!
 
 use crate::type_info::PgTypeKind;
 use crate::{PgTypeInfo, Postgres};