9p: document missing enum values in kernel-doc comments

9p: document missing enum values in kernel-doc comments

Add kernel-doc entries for all undocumented enum values:
- p9_debug_flags: P9_DEBUG_CACHE, P9_DEBUG_MMAP
- p9_msg_t: all 9P2000.L message types (TLOPEN/RLOPEN through
  TUNLINKAT/RUNLINKAT)
- p9_open_mode_t: P9L_MODE_MASK, P9L_DIRECT, P9L_NOWRITECACHE,
  P9L_LOOSE

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Kit Dallege <xaum.io@gmail.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Message-ID: <20260315190633.73536-1-xaum.io@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>