[AttrDocs] Enhance musttail attribute documentation

Based on info from #54964

clang/include/clang/Basic/AttrDocs.td

@@ -603,6 +603,9 @@ Any variables in scope, including all arguments to the function and the
 return value must be trivially destructible. The calling convention of the
 caller and callee must match, and they must not be variadic functions or have
 old style K&R C function declarations.
+
+``clang::musttail`` provides assurances that the tail call can be optimized on
+all targets, not just one.
   }];
 }