i2c: add kdoc for the new debugfs entry of clients

When adding the new debugfs entry, its kdoc equivalent was forgotten.
Add it now.

Fixes: d06905d ("i2c: add core-managed per-client directory in debugfs")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20250115163146.6c48f066@canb.auug.org.au
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

include/linux/i2c.h

@@ -317,6 +317,8 @@ struct i2c_driver {
  *	calls it to pass on slave events to the slave driver.
  * @devres_group_id: id of the devres group that will be created for resources
  *	acquired when probing this device.
+ * @debugfs: pointer to the debugfs subdirectory which the I2C core created
+ *	for this client.
  *
  * An i2c_client identifies a single device (i.e. chip) connected to an
  * i2c bus. The behaviour exposed to Linux is defined by the driver