iio: buffer: buffer_impl.h: fix kernel-doc warnings

iio: buffer: buffer_impl.h: fix kernel-doc warnings

Resolve all kernel-doc warnings in buffer_impl.h:

Warning: include/linux/iio/buffer_impl.h:172 struct member 'direction' not described in 'iio_buffer'
Warning: include/linux/iio/buffer_impl.h:184 No description found for return value of 'iio_update_buffers'

Also correct one typo (word order switch) and remove one stray space
in a kernel-doc comment.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>