[media] DocBook: add entry IDs for enum fe_sec_tone_mode

enum fe_sec_tone_mode is documented together with FE_SET_TONE.

Add xrefs for each entry there. This makes the hyperlinks at
frontend.h to go directly to the right documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>