xen/xenbus: client: fix kernel-doc comments

Correct function kernel-doc notation to prevent warnings from
scripts/kernel-doc.

xenbus_client.c:134: warning: No description found for return value of 'xenbus_watch_path'
xenbus_client.c:177: warning: No description found for return value of 'xenbus_watch_pathfmt'
xenbus_client.c:258: warning: missing initial short description on line:
 * xenbus_switch_state
xenbus_client.c:267: warning: No description found for return value of 'xenbus_switch_state'
xenbus_client.c:308: warning: missing initial short description on line:
 * xenbus_dev_error
xenbus_client.c:327: warning: missing initial short description on line:
 * xenbus_dev_fatal
xenbus_client.c:350: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Equivalent to xenbus_dev_fatal(dev, err, fmt, args), but helps
xenbus_client.c:457: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Allocate an event channel for the given xenbus_device, assigning the newly
xenbus_client.c:486: warning: expecting prototype for Free an existing event channel. Returns 0 on success or(). Prototype was for xenbus_free_evtchn() instead
xenbus_client.c:502: warning: missing initial short description on line:
 * xenbus_map_ring_valloc
xenbus_client.c:517: warning: No description found for return value of 'xenbus_map_ring_valloc'
xenbus_client.c:602: warning: missing initial short description on line:
 * xenbus_unmap_ring
xenbus_client.c:614: warning: No description found for return value of 'xenbus_unmap_ring'
xenbus_client.c:715: warning: missing initial short description on line:
 * xenbus_unmap_ring_vfree
xenbus_client.c:727: warning: No description found for return value of 'xenbus_unmap_ring_vfree'
xenbus_client.c:919: warning: missing initial short description on line:
 * xenbus_read_driver_state
xenbus_client.c:926: warning: No description found for return value of 'xenbus_read_driver_state'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20231206181724.27767-1-rdunlap@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>