docs: ABI: auxdisplay: document linedisp library sysfs attributes

docs: ABI: auxdisplay: document linedisp library sysfs attributes

Add ABI documentation for sysfs attributes provided by the line-display
auxdisplay library module. These attributes enable text message display and
configuration on character-based auxdisplay devices.

Documents previously undocumented attributes:
- message, scroll_step_ms (introduced in v5.16)
- map_seg7, map_seg14 (introduced in v6.9)

The line-display library is used by multiple auxdisplay drivers and
can expose these attributes either on linedisp.N child devices or
directly on parent auxdisplay devices.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>