added missing notNull() case to update kdocs and fixed docs for notNull()

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/update.kt

@@ -3,6 +3,7 @@ package org.jetbrains.kotlinx.dataframe.api
 import org.jetbrains.kotlinx.dataframe.AnyRow
 import org.jetbrains.kotlinx.dataframe.ColumnExpression
 import org.jetbrains.kotlinx.dataframe.ColumnsSelector
+import org.jetbrains.kotlinx.dataframe.DataColumn
 import org.jetbrains.kotlinx.dataframe.DataFrame
 import org.jetbrains.kotlinx.dataframe.DataFrameExpression
 import org.jetbrains.kotlinx.dataframe.DataRow
@@ -11,6 +12,7 @@ import org.jetbrains.kotlinx.dataframe.RowValueFilter
 import org.jetbrains.kotlinx.dataframe.annotations.AccessApiOverload
 import org.jetbrains.kotlinx.dataframe.annotations.Interpretable
 import org.jetbrains.kotlinx.dataframe.annotations.Refine
+import org.jetbrains.kotlinx.dataframe.api.mean
 import org.jetbrains.kotlinx.dataframe.columns.ColumnGroup
 import org.jetbrains.kotlinx.dataframe.columns.ColumnReference
 import org.jetbrains.kotlinx.dataframe.columns.toColumnSet
@@ -78,6 +80,9 @@ public class Update<T, C>(
      * `\[ `__`.`__[**`at`**][Update.at]**`(`**[`rowIndices`][CommonUpdateAtFunctionDoc.RowIndicesParam]**`)`**` ]`
      *
      * {@include [Indent]}
+     * `\[ `__`.`__[**`notNull`**][Update.notNull]**`()`**` ]`
+     *
+     * {@include [Indent]}
      * __`.`__[**`with`**][Update.with]**`  {  `**[`rowExpression`][ExpressionsGivenRow.RowValueExpression.WithExample]**` }`**
      *
      * {@include [Indent]}
@@ -403,7 +408,21 @@ internal infix fun <T, C> RowValueFilter<T, C>?.and(other: RowValueFilter<T, C>)
     return { thisExp(this, it) && other(this, it) }
 }
 
-/** @include [Update.notNull] */
+/**
+ * ## Not Null
+ * Filters the update-selection to only include cells where the value is not null.
+ *
+ * This is shorthand for `.`[where][Update.where]` { it != null }`.
+ *
+ * For example:
+ *
+ * `df.`[update][update]` { `[colsOf][colsOf]`<`[Int][Int]`?>() }.`[notNull][notNull]`().`[perRowCol][Update.perRowCol]` { row, col ->`
+ *
+ * {@include [Indent]}`row\[col\] / col.`[mean][DataColumn.mean]`(skipNA = true)`
+ *
+ * `}`
+ */
+@Suppress("UNCHECKED_CAST")
 @Interpretable("UpdateNotNullDefault")
 public fun <T, C> Update<T, C?>.notNull(): Update<T, C> = where { it != null } as Update<T, C>