docs: xforms_lists: better evaluate struct_group macros

The previous approach were to unwind nested structs/unions.

Now that we have a logic that can handle it well, use it to
ensure that struct_group macros will properly reflect the
actual struct.

Note that the replacemend logic still simplifies the code
a little bit, as the basic build block for struct group is:

union { \
struct { MEMBERS } ATTRS; \
struct __struct_group_tag(TAG) { MEMBERS } ATTRS NAME; \
} ATTRS

There:

- ATTRS is meant to add extra macro attributes like __packed
which we already discard, as they aren't relevant to
document struct members;

- TAG is used only when built with __cplusplus.

So, instead, convert them into just:

struct { MEMBERS };

Please notice that here, we're using the greedy version of the
backrefs, as MEMBERS is actually MEMBERS... on all such macros.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <24bf2c036b08814d9b4aabc27542fd3b2ff54424.1773770483.git.mchehab+huawei@kernel.org>