Add a sample reference to isBlank

runtime/src/main/kotlin/kotlin/text/Strings.kt

@@ -92,6 +92,8 @@ public actual fun String.decapitalize(): String {
 
 /**
  * Returns `true` if this string is empty or consists solely of whitespace characters.
+ *
+ * @sample samples.text.Strings.stringIsBlank
  */
 public actual fun CharSequence.isBlank(): Boolean = length == 0 || indices.all { this[it].isWhitespace() }