KT-38817 capitalize uses title case for the first char where available

This unifies its behavior with new capitalize overload with Locale.

Co-authored-by: Abduqodiri Qurbonzoda <abduqodiri.qurbonzoda@jetbrains.com>

libraries/stdlib/jvm/src/kotlin/text/StringsJVM.kt

@@ -569,19 +569,24 @@ public inline fun String.toPattern(flags: Int = 0): java.util.regex.Pattern {
 }
 
 /**
- * Returns a copy of this string having its first letter uppercased, or the original string,
- * if it's empty or already starts with an upper case letter.
+ * Returns a copy of this string having its first letter titlecased using the rules of the default locale,
+ * or the original string if it's empty or already starts with a title case letter.
+ *
+ * The title case of a character is usually the same as its upper case with several exceptions.
+ * The particular list of characters with the special title case form depends on the underlying platform.
  *
  * @sample samples.text.Strings.capitalize
  */
 public actual fun String.capitalize(): String {
-    return if (isNotEmpty() && this[0].isLowerCase()) substring(0, 1).toUpperCase() + substring(1) else this
+    return capitalize(Locale.getDefault())
 }
 
 /**
- * Returns a copy of this string having its first letter titlecased preferring [Char.toTitleCase] (if different from
- * [Char.toUpperCase]) or by [String.toUpperCase] using the specified [locale], or the original string,
- * if it's empty or already starts with an upper case letter.
+ * Returns a copy of this string having its first letter titlecased using the rules of the specified [locale],
+ * or the original string if it's empty or already starts with a title case letter.
+ *
+ * The title case of a character is usually the same as its upper case with several exceptions.
+ * The particular list of characters with the special title case form depends on the underlying platform.
  */
 @SinceKotlin("1.4")
 @WasExperimental(ExperimentalStdlibApi::class)
@@ -605,17 +610,17 @@ public fun String.capitalize(locale: Locale): String {
 }
 
 /**
- * Returns a copy of this string having its first letter lowercased, or the original string,
- * if it's empty or already starts with a lower case letter.
+ * Returns a copy of this string having its first letter lowercased using the rules of the default locale,
+ * or the original string if it's empty or already starts with a lower case letter.
  *
  * @sample samples.text.Strings.decapitalize
  */
 public actual fun String.decapitalize(): String {
-    return if (isNotEmpty() && this[0].isUpperCase()) substring(0, 1).toLowerCase() + substring(1) else this
+    return if (isNotEmpty() && !this[0].isLowerCase()) substring(0, 1).toLowerCase() + substring(1) else this
 }
 
 /**
- * Returns a copy of this string having its first letter lowercased using the specified [locale],
+ * Returns a copy of this string having its first letter lowercased using the rules of the specified [locale],
  * or the original string, if it's empty or already starts with a lower case letter.
  */
 @SinceKotlin("1.4")