docs: kdoc: remove the inline states-within-a-state

docs: kdoc: remove the inline states-within-a-state

The processing of inline kerneldoc comments is a state like the rest, but
it was implemented as a set of separate substates.  Just remove the
substate logic and make the inline states normal ones like the rest.

INLINE_ERROR was never actually used for anything, so just take it out.

No changes to the generated output.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250627184000.132291-8-corbet@lwn.net