kernel-doc: handle #if in enums as well

In addition to #ifdef, #define and #endif, also handle
any #if since we may be using e.g. #if IS_ENABLED(...).

I didn't find any instances of this in the kernel now,
there are enums with such ifs inside, but I didn't find
any with kernel-doc as well. However, it came up as we
were adding such a construct in our driver and warnings
from kernel-doc were the result.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240214142937.80ee86a3beae.Ibcc5bd97a20cd10a792663e4b254cd46c7e8b520@changeid