KT-20357: Add samples for indexOf

libraries/stdlib/samples/test/samples/text/strings.kt

@@ -204,6 +204,7 @@ class Strings {
         val noPadding = "abcde".padEnd(3)
         assertPrints("'$noPadding'", "'abcde'")
     }
+
     @Sample
     fun clearStringBuilder() {
         val builder = StringBuilder()
@@ -357,4 +358,15 @@ class Strings {
         assertPrints(string.map { it.toUpperCase() }, "[K, O, T, L, I, N]")
     }
 
+    @Sample
+    fun indexOf() {
+        fun getSubstringDetails(str: String, toFind: String): String {
+            return "Found $toFind in $str at position ${str.indexOf(toFind)}"
+        }
+
+        val str = "Kotlin"
+        val toFind = "otli"
+
+        assertPrints(getSubstringDetails(str, toFind), "Found otli in Kotlin at position 1")
+    }
 }

libraries/stdlib/src/kotlin/text/Strings.kt

@@ -984,6 +984,7 @@ public fun CharSequence.lastIndexOfAny(strings: Collection<String>, startIndex:
  *
  * @param ignoreCase `true` to ignore character case when matching a character. By default `false`.
  * @return An index of the first occurrence of [char] or -1 if none is found.
+ * @sample samples.text.Strings.indexOf
  */
 public fun CharSequence.indexOf(char: Char, startIndex: Int = 0, ignoreCase: Boolean = false): Int {
     return if (ignoreCase || this !is String)
@@ -998,6 +999,7 @@ public fun CharSequence.indexOf(char: Char, startIndex: Int = 0, ignoreCase: Boo
  *
  * @param ignoreCase `true` to ignore character case when matching a string. By default `false`.
  * @return An index of the first occurrence of [string] or `-1` if none is found.
+ * @sample samples.text.Strings.indexOf
  */
 public fun CharSequence.indexOf(string: String, startIndex: Int = 0, ignoreCase: Boolean = false): Int {
     return if (ignoreCase || this !is String)