Improve docs for substring/subsequence.

core/builtins/native/kotlin/CharSequence.kt

@@ -26,12 +26,13 @@ public interface CharSequence {
     public val length: Int
 
     /**
-     * Returns the character at the specified [index] in the sequence.
+     * Returns the character at the specified [index] in this character sequence.
      */
     public operator fun get(index: Int): Char
 
     /**
-     * Returns a subsequence of this sequence.
+     * Returns a new character sequence that is a subsequence of this character sequence,
+     * starting at the specified [startIndex] and ending right before the specified [endIndex].
      *
      * @param startIndex the start index (inclusive).
      * @param endIndex the end index (exclusive).

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

@@ -277,23 +277,26 @@ public fun CharSequence.hasSurrogatePairAt(index: Int): Boolean {
 }
 
 /**
- * Returns a substring specified by the given [range].
+ * Returns a substring specified by the given [range] of indices.
  */
 public fun String.substring(range: IntRange): String = substring(range.start, range.endInclusive + 1)
 
 /**
- * Returns a subsequence of this char sequence specified by the given [range].
+ * Returns a subsequence of this char sequence specified by the given [range] of indices.
  */
 public fun CharSequence.subSequence(range: IntRange): CharSequence = subSequence(range.start, range.endInclusive + 1)
 
 /**
- * Returns a substring of chars from a range of this char sequence specified by [startIndex] and [endIndex] indices.
+ * Returns a substring of chars from a range of this char sequence starting at the [startIndex] and ending right before the [endIndex].
+ *
+ * @param startIndex the start index (inclusive).
+ * @param endIndex the end index (exclusive). If not specified, the length of the char sequence is used.
  */
 @kotlin.internal.InlineOnly
 public inline fun CharSequence.substring(startIndex: Int, endIndex: Int = length): String = subSequence(startIndex, endIndex).toString()
 
 /**
- * Returns a substring of chars from a [range] of this char sequence.
+ * Returns a substring of chars at indices from the specified [range] of this char sequence.
  */
 public fun CharSequence.substring(range: IntRange): String = subSequence(range.start, range.endInclusive + 1).toString()
 

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

@@ -156,13 +156,16 @@ public fun CharSequence.split(regex: Pattern, limit: Int = 0): List<String>
 }
 
 /**
- * Returns a substring of this string starting with the specified index.
+ * Returns a substring of this string that starts at the specified [startIndex] and continues to the end of the string.
  */
 @kotlin.internal.InlineOnly
 public inline fun String.substring(startIndex: Int): String = (this as java.lang.String).substring(startIndex)
 
 /**
- * Returns the substring of this string starting and ending at the specified indices.
+ * Returns the substring of this string starting at the [startIndex] and ending right before the [endIndex].
+ *
+ * @param startIndex the start index (inclusive).
+ * @param endIndex the end index (exclusive).
  */
 @kotlin.internal.InlineOnly
 public inline fun String.substring(startIndex: Int, endIndex: Int): String = (this as java.lang.String).substring(startIndex, endIndex)