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

libraries/stdlib/test/text/CharTest.kt

@@ -17,6 +17,8 @@ class CharTest {
             "Κκϰ", "Ππϖ", "Ρρϱ", "Σςσ", "Φφϕ", "ΩωΩ", "Ṡṡẛ",
             "Θθϑϴ", "Iiİı",
         )
+
+       val allCharsByCategory = (Char.MIN_VALUE..Char.MAX_VALUE).groupBy { it.category }
     }
 
     @Test
@@ -326,6 +328,17 @@ class CharTest {
         assertEquals("Cc", CharCategory.CONTROL.code)
     }
 
+    @Test
+    fun whitespace() {
+        val allWhitespace = (Char.MIN_VALUE..Char.MAX_VALUE).filter { it.isWhitespace() }
+        val expected =
+            listOf(CharCategory.SPACE_SEPARATOR, CharCategory.LINE_SEPARATOR, CharCategory.PARAGRAPH_SEPARATOR)
+                .flatMap { allCharsByCategory[it]!! } +
+                    ('\u0009'..'\u000D') +
+                    ('\u001C'..'\u001F')
+        assertEquals(expected.sorted(), allWhitespace)
+    }
+
     @Test
     fun lowercaseChar() {
         assertEquals('\u0000', '\u0000'.lowercaseChar())