[SPARK-24408][SQL][DOC] Move abs function to math_funcs group

## What changes were proposed in this pull request?

A few math functions (`abs` , `bitwiseNOT`, `isnan`, `nanvl`) are not in **math_funcs** group. They should really be.

## How was this patch tested?

Awaiting Jenkins

Author: Jacek Laskowski <[email protected]>

Closes #21448 from jaceklaskowski/SPARK-24408-math-funcs-doc.

Author: jaceklaskowski

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -1031,14 +1031,6 @@ object functions {
   // Non-aggregate functions
   //////////////////////////////////////////////////////////////////////////////////////////////
 
-  /**
-   * Computes the absolute value.
-   *
-   * @group normal_funcs
-   * @since 1.3.0
-   */
-  def abs(e: Column): Column = withExpr { Abs(e.expr) }
-
   /**
    * Creates a new array column. The input columns must all have the same data type.
    *
@@ -1336,7 +1328,7 @@ object functions {
   }
 
   /**
-   * Computes bitwise NOT.
+   * Computes bitwise NOT (~) of a number.
    *
    * @group normal_funcs
    * @since 1.4.0
@@ -1364,6 +1356,14 @@ object functions {
   // Math Functions
   //////////////////////////////////////////////////////////////////////////////////////////////
 
+  /**
+   * Computes the absolute value of a numeric value.
+   *
+   * @group math_funcs
+   * @since 1.3.0
+   */
+  def abs(e: Column): Column = withExpr { Abs(e.expr) }
+
   /**
    * @return inverse cosine of `e` in radians, as if computed by `java.lang.Math.acos`
    *