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/js/src/kotlin/text/stringsCode.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -70,8 +70,11 @@ public actual fun CharSequence.regionMatches(thisOffset: Int, other: CharSequenc
 
 
 /**
- * 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
  */
@@ -80,8 +83,8 @@ public actual fun String.capitalize(): 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
  */