Rename sgn to sign, add docs and tests

#KT-4900

libraries/stdlib/src/kotlin/util/MathJVM.kt

@@ -317,13 +317,25 @@ public inline fun round(a: Double): Double = nativeMath.rint(a)
 /**
  * Returns the absolute value of the given value [a].
  *
+ * Special cases:
+ *     - `abs(NaN)` is `NaN`
+ *
  * @see absoluteValue extension property for [Double]
  */
 @InlineOnly
 public inline fun abs(a: Double): Double = nativeMath.abs(a)
-// also as extension val [sign]
+
+/**
+ * Returns the sign of the given value [a]:
+ *     - `-1.0` if the value is negative,
+ *     - zero if the value is zero,
+ *     - `1.0` if the value is positive
+ *
+ * Special case:
+ *     - `sign(NaN)` is `NaN`
+ */
 @InlineOnly
-public inline fun sgn(a: Double): Double = nativeMath.signum(a)
+public inline fun sign(a: Double): Double = nativeMath.signum(a)
 
 
 
@@ -364,10 +376,41 @@ public inline fun Double.pow(other: Int): Double = nativeMath.pow(this, other.to
 
 
 public inline fun Double.IEEErem(other: Double): Double = nativeMath.IEEEremainder(this, other)
+
+/**
+ * Returns the absolute value of this value.
+ *
+ * Special cases:
+ *     - `NaN.absoluteValue` is `NaN`
+ *
+ * @see abs function
+ */
+@InlineOnly
 public inline val Double.absoluteValue: Double get() = nativeMath.abs(this)
+
+/**
+ * Returns the sign of this value:
+ *     - `-1.0` if the value is negative,
+ *     - zero if the value is zero,
+ *     - `1.0` if the value is positive
+ *
+ * Special case:
+ *     - `NaN.sign` is `NaN`
+ */
+@InlineOnly
 public inline val Double.sign: Double get() = nativeMath.signum(this)
 
+/**
+ * Returns this value with the sign bit same as of the [sign] value.
+ *
+ * If [sign] is `NaN` the sign of the result is undefined.
+ */
+@InlineOnly
 public inline fun Double.withSign(sign: Double): Double = nativeMath.copySign(this, sign)
+/**
+ * Returns this value with the sign bit same as of the [sign] value.
+ */
+@InlineOnly
 public inline fun Double.withSign(sign: Int): Double = nativeMath.copySign(this, sign.toDouble())
 
 public inline val Double.ulp: Double get() = nativeMath.ulp(this)