timer: optimize to gi-docgen 2024.1 or later, approximately

As long as building with gi-docgen 2025.3.1 in Ubuntu 25.10, some
annotations for SlaveClass become invalid and causes some warnings.

/usr/bin/gi-docgen generate --no-namespace-dir --config=doc/timer/alsatimer.toml --output-dir=doc/timer/alsatimer --content-dir=/home/mocchi/git/alsa-project.org/alsa-gobject/doc/timer src/timer/ALSATimer-0.0.gir
WARNING: Unknown namespace SlaveClass
slave_id. If the slave_class is [enum@SlaveClass.APPLICATION], the slave_id is for the PID of
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
WARNING: Unknown namespace SlaveClass
[enum@SlaveClass.SEQUENCER], the slave_id is the numeric ID of queue bound for timer device.
^~~~~~~~~~~~~~~~~~~~~~~~~~~
WARNING: Unknown namespace SlaveClass
slave_id. If the slave_class is [enum@SlaveClass.APPLICATION], the slave_id is for the PID of
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
WARNING: Unknown namespace SlaveClass
[enum@SlaveClass.SEQUENCER], the slave_id is the numeric ID of queue bound for timer device.
^~~~~~~~~~~~~~~~~~~~~~~~~~~
WARNING: Unknown namespace SlaveClass
slave_id. If the slave_class is [enum@SlaveClass.APPLICATION], the slave_id is for the PID of
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
WARNING: Unknown namespace SlaveClass
[enum@SlaveClass.SEQUENCER], the slave_id is the numeric ID of queue bound for timer device.
^~~~~~~~~~~~~~~~~~~~~~~~~~~

This commit suppresses the warnings by using the pattern of enum@class.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

Author: takaswie

src/timer/user-instance.c

@@ -332,9 +332,9 @@ gboolean alsatimer_user_instance_attach(ALSATimerUserInstance *self, ALSATimerDe
  * @error: A [struct@GLib.Error]. Error is generated with domain of `ALSATimer.UserInstanceError`.
  *
  * Attach the instance as an slave to another instance indicated by a pair of slave_class and
- * slave_id. If the slave_class is [enum@SlaveClass:APPLICATION], the slave_id is for the PID of
+ * slave_id. If the slave_class is [enum@SlaveClass].APPLICATION, the slave_id is for the PID of
  * application process which owns the instance of timer. If the slave_class is
- * [enum@SlaveClass:SEQUENCER], the slave_id is the numeric ID of queue bound for timer device.
+ * [enum@SlaveClass].SEQUENCER, the slave_id is the numeric ID of queue bound for timer device.
  *
  * The call of function executes `ioctl(2)` system call with `SNDRV_TIMER_IOCTL_SELECT` command
  * for ALSA timer character device.