add_library: mention that Xcode requires a real source file

The docs mentioned that it happened, but not when. Mention Xcode since
it is the culprit today.

See #16524.

Help/command/add_library.rst

@@ -111,9 +111,9 @@ may contain only sources that compile, header files, and other files
 that would not affect linking of a normal library (e.g. ``.txt``).
 They may contain custom commands generating such sources, but not
 ``PRE_BUILD``, ``PRE_LINK``, or ``POST_BUILD`` commands.  Object libraries
-cannot be linked.  Some native build systems may not like targets that
-have only object files, so consider adding at least one real source file
-to any target that references ``$<TARGET_OBJECTS:objlib>``.
+cannot be linked.  Some native build systems (such as Xcode) may not like
+targets that have only object files, so consider adding at least one real
+source file to any target that references ``$<TARGET_OBJECTS:objlib>``.
 
 Alias Libraries
 ^^^^^^^^^^^^^^^