update docs for add

Author: koperagen

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api/Modify.kt

@@ -821,6 +821,22 @@ class Modify : TestBase() {
         // SampleEnd
     }
 
+    @Test
+    fun addCalculated() {
+        class CityInfo(val city: String?, val population: Int, val location: String)
+        fun queryCityInfo(city: String?): CityInfo { return CityInfo(city, city?.length ?: 0, "35.5 32.2") }
+        // SampleStart
+        val personWithCityInfo = df.add {
+            val cityInfo = city.map { queryCityInfo(it) }
+            "cityInfo" {
+                cityInfo.map { it.location } into CityInfo::location
+                cityInfo.map { it.population } into "population"
+            }
+        }
+        // SampleEnd
+        personWithCityInfo["cityInfo"]["population"] shouldBe df.city.map { it?.length ?: 0 }.named("population")
+    }
+
     @Test
     fun addMany_properties() {
         // SampleStart

docs/StardustDocs/topics/add.md

@@ -64,7 +64,14 @@ add {
     ...
 }
 
-columnMapping = column into columnName | columnName from column | columnName from { rowExpression }
+columnMapping = column into columnName 
+    | columnName from column 
+    | columnName from { rowExpression }
+    | columnGroupName { 
+        columnMapping
+        columnMapping
+        ...
+    }
 ```
 
 <!---FUN addMany-->
@@ -146,6 +153,22 @@ df.add(df1, df2)
 
 <!---END-->
 
+**Add columns using intermediate result:**
+
+<!---FUN addCalculated-->
+
+```kotlin
+val personWithCityInfo = df.add {
+    val cityInfo = city.map { queryCityInfo(it) }
+    "cityInfo" {
+        cityInfo.map { it.location } into CityInfo::location
+        cityInfo.map { it.population } into "population"
+    }
+}
+```
+
+<!---END-->
+
 ## addId
 
 Adds column with sequential values 0, 1, 2,... New column will be added in the beginning of columns list and will become the first column in `DataFrame`.