[Clang] Clarify the [[trivial_abi]] documentation. (#143243)

Fixes #36667

Author: cor3ntin

clang/include/clang/Basic/AttrDocs.td

@@ -4433,7 +4433,14 @@ destroy the object before returning. The lifetime of the copy of the parameter
 in the caller ends without a destructor call when the call begins.
 
 If a type is trivial for the purpose of calls, it is assumed to be trivially
-relocatable for the purpose of ``__is_trivially_relocatable``.
+relocatable for the purpose of ``__is_trivially_relocatable`` and
+``__builtin_is_cpp_trivially_relocatable``.
+When a type marked with ``[[trivial_abi]]`` is used as a function argument,
+the compiler may omit the call to the copy constructor.
+Thus, side effects of the copy constructor are potentially not performed.
+For example, objects that contain pointers to themselves or otherwise depend
+on their address (or the address or their subobjects) should not be declared
+``[[trivial_abi]]``.
 
 Attribute ``trivial_abi`` has no effect in the following cases: