net/iucv: clean up iucv kernel-doc warnings

Fix numerous (many) kernel-doc warnings in iucv.[ch]:

- convert function documentation comments to a common (kernel-doc) look,
  even for static functions (without "/**")
- use matching parameter and parameter description names
- use better wording in function descriptions (Jakub & AI)
- remove duplicate kernel-doc comments from the header file (Jakub)

Examples:

Warning: include/net/iucv/iucv.h:210 missing initial short description
 on line: * iucv_unregister
Warning: include/net/iucv/iucv.h:216 function parameter 'handle' not
 described in 'iucv_unregister'
Warning: include/net/iucv/iucv.h:467 function parameter 'answer' not
 described in 'iucv_message_send2way'
Warning: net/iucv/iucv.c:727 missing initial short description on line:
 * iucv_cleanup_queue

Build-tested with both "make htmldocs" and "make ARCH=s390 defconfig all".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20260203075248.1177869-1-rdunlap@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>