docs: kdoc: rework the rewrite_struct_members() main loop

docs: kdoc: rework the rewrite_struct_members() main loop

Adopt a more Pythonic form for the main loop of this function, getting rid
of the "while True:" construction and making the actual loop invariant
explicit.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250807211639.47286-8-corbet@lwn.net