[stdlib] Fixes to doc. for formIndex of Indexable

Added documentation description for third parameter of

    formIndex(_:offsetBy:limitedBy:)

which was previously omitted.

Description copied from the 'index(_:offsetBy:limitedBy:)' method.

Author: dfrib

stdlib/public/core/Collection.swift

@@ -281,6 +281,9 @@ public protocol _Indexable : _IndexableBase {
   ///   - i: A valid index of the collection.
   ///   - n: The distance to offset `i`. `n` must not be negative unless the
   ///     collection conforms to the `BidirectionalCollection` protocol.
+  ///   - limit: A valid index of the collection to use as a limit. If `n > 0`,
+  ///     a limit that is less than `i` has no effect. Likewise, if `n < 0`, a
+  ///     limit that is greater than `i` has no effect.
   /// - Returns: `true` if `i` has been offset by exactly `n` steps without
   ///   going beyond `limit`; otherwise, `false`. When the return value is
   ///   `false`, the value of `i` is equal to `limit`.
@@ -1037,6 +1040,9 @@ extension _Indexable {
   ///   - i: A valid index of the collection.
   ///   - n: The distance to offset `i`. `n` must not be negative unless the
   ///     collection conforms to the `BidirectionalCollection` protocol.
+  ///   - limit: A valid index of the collection to use as a limit. If `n > 0`,
+  ///     a limit that is less than `i` has no effect. Likewise, if `n < 0`, a
+  ///     limit that is greater than `i` has no effect.
   /// - Returns: `true` if `i` has been offset by exactly `n` steps without
   ///   going beyond `limit`; otherwise, `false`. When the return value is
   ///   `false`, the value of `i` is equal to `limit`.