Match docs to current behavior.

Confirmed this change on using the swift-5.2-DEVELOPMENT-SNAPSHOT-2020-01-22-a
toolchain, Apple Swift version 5.2-dev (Swift d3f0448).

Fixes rdar://problem/58717942

Author: amartini51

stdlib/public/core/Array.swift

@@ -432,7 +432,7 @@ extension Array: _ArrayProtocol {
   ///
   ///     numbers.append(contentsOf: stride(from: 60, through: 100, by: 10))
   ///     // numbers.count == 10
-  ///     // numbers.capacity == 12
+  ///     // numbers.capacity == 10
   @inlinable
   public var capacity: Int {
     return _getCapacity()

stdlib/public/core/ArraySlice.swift

@@ -249,7 +249,7 @@ extension ArraySlice: _ArrayProtocol {
   ///
   ///     numbers.append(contentsOf: stride(from: 60, through: 100, by: 10))
   ///     // numbers.count == 10
-  ///     // numbers.capacity == 12
+  ///     // numbers.capacity == 10
   @inlinable
   public var capacity: Int {
     return _getCapacity()

stdlib/public/core/ContiguousArray.swift

@@ -120,7 +120,7 @@ extension ContiguousArray: _ArrayProtocol {
   ///
   ///     numbers.append(contentsOf: stride(from: 60, through: 100, by: 10))
   ///     // numbers.count == 10
-  ///     // numbers.capacity == 12
+  ///     // numbers.capacity == 10
   @inlinable
   public var capacity: Int {
     return _getCapacity()