[media] dvb: frontend.h: improve dvb_frontent_parameters comment

The comment for struct dvb_frontend_parameters is weird, as it
mixes delivery system name (ATSC) with modulation names
(QPSK, QAM, OFDM).

Use delivery system names there on the frequency comment, as this
is clearer, specially after 2GEN delivery systems.

While here, add comments at the union, to make live easier for ones
that may try to understand the convention used by the legacy API.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

include/uapi/linux/dvb/frontend.h

···
 };
 
 struct dvb_frontend_parameters {
-	__u32 frequency;     /* (absolute) frequency in Hz for QAM/OFDM/ATSC */
-			     /* intermediate frequency in kHz for QPSK */
+	__u32 frequency;     /* (absolute) frequency in Hz for DVB-C/DVB-T/ATSC */
+			     /* intermediate frequency in kHz for DVB-S */
 	fe_spectral_inversion_t inversion;
 	union {
-		struct dvb_qpsk_parameters qpsk;
-		struct dvb_qam_parameters  qam;
-		struct dvb_ofdm_parameters ofdm;
-		struct dvb_vsb_parameters vsb;
+		struct dvb_qpsk_parameters qpsk;	/* DVB-S */
+		struct dvb_qam_parameters  qam;		/* DVB-C */
+		struct dvb_ofdm_parameters ofdm;	/* DVB-T */
+		struct dvb_vsb_parameters vsb;		/* ATSC */
 	} u;
 };