container_of: Document container_of() is not to be used in new code

container_of: Document container_of() is not to be used in new code

There is a warning in the kerneldoc documentation of container_of() that
constness of its ptr argument is lost. While this is a valid suggestion
container_of_const() should be used instead, the vast majority of new
code still uses container_of():

$ git diff v6.13 v6.14|grep container_of\(|wc -l
646
$ git diff v6.13 v6.14|grep container_of_const|wc -l
9

Make an explicit recommendation to use container_of_const().

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://lore.kernel.org/r/20250520103437.468691-1-sakari.ailus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>