[stdlib] Improve documentation of Char.isWhitespace and test it behaves the same in different targets

libraries/stdlib/native-wasm/src/kotlin/text/Char.kt

@@ -74,7 +74,12 @@ public actual fun Char.isDigit(): Boolean {
 }
 
 /**
- * Determines whether a character is whitespace according to the Unicode standard.
+ * Determines whether a character is whitespace.
+ *
+ * A character is considered whitespace if either its Unicode [category][Char.category]
+ * is one of [CharCategory.SPACE_SEPARATOR], [CharCategory.LINE_SEPARATOR], [CharCategory.PARAGRAPH_SEPARATOR],
+ * or it is a [CharCategory.CONTROL] character in range `U+0009..U+000D` or `U+001C..U+001F`.
+ *
  * Returns `true` if the character is whitespace.
  *
  * @sample samples.text.Chars.isWhitespace