[cxx-interop] Document some caveats of safe interop

Some of these like the compiler flag to enable the safe wrappers is not
documented anywhere. Having this documentation in place can help more
people to play around with the feature and report bugs.

Author: Gabor Horvath

documentation/cxx-interop/safe-interop/index.md

@@ -105,6 +105,10 @@ struct SWIFT_NONESCAPABLE StringRef { ... };
 Now the Swift compiler imports `StringRef` as a safe type and no longer
 emits a warning about using an unsafe type.
 
+<div class="info" markdown="1">
+Some containers and protocols do not yet support non-escapable types in Swift 6.2.
+</div>
+
 ### Annotating C++ APIs
 
 Building the code again will emit a new diagnostic for the `fileName` function about
@@ -387,6 +391,13 @@ compiler can bridge those span-like parameters to Swift's
 and [`MutableSpan`](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0467-MutableSpan.md)
 types, and the user with a safe and convenient interface to those imported APIs.
 
+<div class="info" markdown="1">
+At the time of writing, the features described in this section
+are behind an experimental feature flag on the Swift 6.2 release branch.
+To enable these features, pass `-enable-experimental-feature SafeInteropWrappers`
+to the Swift compiler.
+</div>
+
 ### C++ `std::span` Support
 
 APIs taking or returning C++'s `std::span` with sufficient lifetime