[cxx-interop] Add documentation about changes to SWIFT_SHARED_REFERENCE feature in Swift 6.2

[fahadnayyar]

Updating the C++ interop documentation by clarifying ownership and initialization semantics for SWIFT_SHARED_REFERENCE types. This change is documenting:

• Guarantees and assumptions when SWIFT_SHARED_REFERENCE types are passed as parameters from Swift to C++ functions or methods.
• How SWIFT_RETURNS_RETAINED and SWIFT_RETURNS_UNRETAINED can be used to specity ownership of returned values in C++ APIs returning SWIFT_SHARED_REFERENCE types.
• Propagation of foreign reference annotations to inherited C++ types.
• Swift initializer support for C++ constructors and static factory methods of SWIFT_SHARED_REFERENCE types annotated with SWIFT_NAME.

[fahadnayyar]

@shahmishal this patch should now be ready to merged!

CC: @ravikandhadai