Add the build setting in Xcode as another example

Incorporates tech review from Alexis Laferrière <[email protected]>.

Author: amartini51

TSPL.docc/LanguageGuide/AccessControl.md

@@ -51,7 +51,9 @@ as part of configuring the build system you're using,
 not as part of your Swift source code.
 For example, if you use Swift Package Manager to build your code,
 you define a package in your `Package.swift` file
-using APIs from the [PackageDescription][] module.
+using APIs from the [PackageDescription][] module;
+if you use Xcode, you specify the package name
+in the Package Access Identifier build setting.
 
 [PackageDescription](https://developer.apple.com/documentation/packagedescription)