[embedded] Expand ABI docs on C/C++ interop

Author: kubamracek

docs/EmbeddedSwift/ABI.md

@@ -6,10 +6,18 @@
 
 For an introduction and motivation into Embedded Swift, please see "[A Vision for Embedded Swift](https://github.com/apple/swift-evolution/blob/main/visions/embedded-swift.md)", a Swift Evolution document highlighting the main goals and approaches.
 
+## ABI stability
+
+The ABI of code generated by Embedded Swift is not currently stable. For a concrete compiler version, it will be consistent, but do not mix code built with different compiler versions.
+
+Similarly, do not mix Embedded Swift code with full Swift code, as the ABIs are different. Details are described in the following sections.
+
 ## Calling convention of Embedded Swift
 
 As of today, Embedded Swift has identical calling convention to full Swift. However, this does not need to continue in the future, and there should not be expectations that the ABI of Embedded Swift is compatible with full Swift.
 
+The compiler respects the ABIs and calling conventions of C and C++ when interoperating with code in those languages. Calling C/C++ functions from Embedded Swift code is supported, and similarly exporting Swift code via `@_extern`, `@_cdecl` or `@_expose` will match the right calling conventions that C/C++ expects.
+
 ## Metadata ABI of Embedded Swift
 
 Embedded Swift eliminates almost all metadata compared to full Swift. However, class metadata is still used, because those serve as vtables for dynamic dispatch of methods to implement runtime polymorphism. The layout of Embedded Swift's class metadata is *different* from full Swift: