Docs: make sure it's hard to miss that type name has to match

Some people (me) doesn't read the full page or just gloss over the
naming section. It's a lot of wasted time debugging the issue when the
type name is not matching, adding name attribute to the examples
makes it really hard to miss.

Author: vargad

postgres-types/src/lib.rs

@@ -31,6 +31,7 @@
 //!
 //! # #[cfg(feature = "derive")]
 //! #[derive(Debug, ToSql, FromSql)]
+//! // #[postgres(name = "Mood")] /* in case type name is different */
 //! enum Mood {
 //!     Sad,
 //!     Ok,
@@ -52,6 +53,7 @@
 //!
 //! # #[cfg(feature = "derive")]
 //! #[derive(Debug, ToSql, FromSql)]
+//! // #[postgres(name = "SessionId")] /* in case type name is different */
 //! struct SessionId(Vec<u8>);
 //! ```
 //!
@@ -88,6 +90,7 @@
 //!
 //! # #[cfg(feature = "derive")]
 //! #[derive(Debug, ToSql, FromSql)]
+//! // #[postgres(name = "InventoryItem")] /* in case type name is different */
 //! struct InventoryItem {
 //!     name: String,
 //!     supplier_id: i32,