regset: Fix kerneldoc for struct regset_get() in user_regset

regset: Fix kerneldoc for struct regset_get() in user_regset

Commit 7717cb9bdd04 ("regset: new method and helpers for it") added a
new interface ->regset_get() for struct user_regset, and commit
1e6986c9db21 ("regset: kill ->get()") got rid of the old interface.

The kerneldoc comment block was never updated to take account of this
change, though.

Update it.

No functional change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/20250701135616.29630-2-Dave.Martin@arm.com
Signed-off-by: Kees Cook <kees@kernel.org>