Clarify docs on custom representations for database functions.

Author: mbrandonw

Sources/StructuredQueriesSQLiteCore/Documentation.docc/Articles/CustomFunctions.md

@@ -50,8 +50,35 @@ configuration.prepareDatabase { db in
 
 ### Custom representations
 
-To define a type that works with a custom representation, like JSON, you can use the `as` parameter
-of the macro:
+To define a type that works with a custom representation, i.e. anytime you use `@Column(as:)` in
+your data type, you can use the `as` parameter of the macro to specify those types. For example,
+if your model holds onto a date and you want to store that date as a 
+[unix timestamp](<doc:Foundation/Date/UnixTimeRepresentation-struct>) (i.e. double),
+then you can do so like this:
+
+```swift
+@Table
+struct Reminder {
+  let id: UUID
+  var title = ""
+  @Column(as: Date.UnixTimeRepresentation.self)
+  var dueDate: Date
+}
+```
+
+And if you wanted to pass this `dueDate` to a Swift database function, you can do so like this:
+
+```swift
+@DatabaseFunction(
+  as: ((Date.UnixTimeRepresentation.self) -> Bool).self
+)
+func isPastDue(_ date: Date) -> Bool {
+  date < Date()
+}
+```
+
+As another example, if you wanted to pass an array of strings from SQL to your Swift database
+function, then you can shuffle the data through using json:
 
 ```swift
 @DatabaseFunction(