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>

tools/lib/python/kdoc/xforms_lists.py

@@ -61,10 +61,16 @@ class CTransforms:
         (CMatch(r"__acquires_shared"), ""),
         (CMatch(r"__releases_shared"), ""),
 
-        (CMatch('struct_group'), r'\2'),
-        (CMatch('struct_group_attr'), r'\3'),
-        (CMatch('struct_group_tagged'), r'struct \1 \2; \3'),
-        (CMatch('__struct_group'), r'\4'),
+        #
+        # Macro __struct_group() creates an union with an anonymous
+        # and a non-anonymous struct, depending on the parameters. We only
+        # need one of those at kernel-doc, as we won't be documenting the same
+        # members twice.
+        #
+        (CMatch('struct_group'), r'struct { \2+ };'),
+        (CMatch('struct_group_attr'), r'struct { \3+ };'),
+        (CMatch('struct_group_tagged'), r'struct { \3+ };'),
+        (CMatch('__struct_group'), r'struct { \4+ };'),
 
     ]