Update require.md

Author: koperagen

docs/StardustDocs/topics/require.md

@@ -14,6 +14,11 @@ require { column }
 **Related operations**: [](cast.md), [](convertTo)
 
 ```kotlin
+// Before `require` extension property will not be resolved
+// peopleDf.select { name.firstName }
+
+// Require a column with a runtime check
 val df = peopleDf.require { "name"["firstName"]<String>() }
+// Use extension property after `require`
 val v: String = df.name.firstName[0]
 ```