Enhance documentation for features brought by #690

Documentation/SQLInterpolation.md

@@ -137,13 +137,29 @@ For example:
 ```swift
 // SELECT * FROM player WHERE name = 'O''Brien'
 let request = Player.filter(literal: "name = \("O'Brien")")
+```
+
+You can also build literals from other values. For example, let's call the `DATE` SQLite function on a query interface column:
 
+```swift
 // SELECT * FROM "player" WHERE DATE("createdAt") = '2020-01-23'
 let createdAt = Column("createdAt")
 let creationDay = SQLLiteral("DATE(\(createdAt))").sqlExpression
 let request = Player.filter(creationDay == "2020-01-23")
 ```
 
+Such literals can be returned by Swift functions:
+
+```swift
+func date(_ value: SQLExpressible) -> SQLExpression {
+    SQLLiteral("DATE(\(lhs.sqlExpression)").sqlExpression
+}
+
+// SELECT * FROM "player" WHERE DATE("createdAt") = '2020-01-23'
+let createdAt = Column("createdAt")
+let request = Player.filter(date(createdAt) == "2020-01-23")
+```
+
 
 ## SQL Interpolation and Record Protocols