[ObjC] Add comment on import for future reference.

PiperOrigin-RevId: 649106953

objectivec/Tests/GPBSwiftTests.swift

@@ -7,7 +7,10 @@
 
 import Foundation
 import XCTest
+// This works for the Xcode project, but for any other build that gets the tests wired up
+// it will need something else.
 import ProtocolBuffers
+
 // Test some usage of the ObjC library from Swift.
 
 private func dataFromStr(_ str: String) -> Data {