nstree: fix kernel-doc comments for internal functions

nstree: fix kernel-doc comments for internal functions

Documentation build reported:

  Warning: kernel/nstree.c:325 function parameter 'ns_tree' not described in '__ns_tree_adjoined_rcu'
  Warning: kernel/nstree.c:325 expecting prototype for ns_tree_adjoined_rcu(). Prototype was for __ns_tree_adjoined_rcu() instead
  Warning: kernel/nstree.c:353 expecting prototype for ns_tree_gen_id(). Prototype was for __ns_tree_gen_id() instead

The kernel-doc comments for `__ns_tree_adjoined_rcu()` and
`__ns_tree_gen_id()` had mismatched function names and a missing
parameter description. This patch updates the function names in the
kernel-doc headers and adds the missing `@ns_tree` parameter description
for `__ns_tree_adjoined_rcu()`.

Fixes: 885fc8ac0a4d ("nstree: make iterator generic")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511061542.0LO7xKs8-lkp@intel.com
Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Link: https://patch.msgid.link/20251111112533.2254432-1-kriish.sharma2006@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>