media: docs-next: update the fe_status documentation for FE_NONE

Recently added FE_NONE to the enum fe_status, so update the
documentation accordingly.

[mchehab@s-opensource.com: change description to actually
 reflect what FE_NONE means: no lock of any kind]
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>