Make some of the fields comments more clear and specific.

Test: Existing unit tests still pass.

Change-Id: I65f015255958b29a44a7f844560694518e01212b
diff --git a/gnss/1.0/IGnssCallback.hal b/gnss/1.0/IGnssCallback.hal
index eb66d78..afef55f 100644
--- a/gnss/1.0/IGnssCallback.hal
+++ b/gnss/1.0/IGnssCallback.hal
@@ -111,9 +111,16 @@
 
         /*
          * Carrier frequency of the signal tracked, for example it can be the
-         * GPS L1 = 1.57542e9 Hz, or L2, L5, varying GLO channels, etc. If
-         * the field is not set, it is the primary common use frequency,
-         * e.g. L1 for GPS.
+         * GPS central frequency for L1 = 1575.45 MHz, or L2 = 1227.60 MHz, L5 =
+         * 1176.45 MHz, varying GLO channels, etc. If the field is not set, it
+         * is the primary common use central frequency, e.g. L1 = 1575.45 MHz
+         * for GPS.
+         *
+         * For an L1, L5 receiver tracking a satellite on L1 and L5 at the same
+         * time, two GnssSvInfo structs must be reported for this same
+         * satellite, in one of the structs, all the values related
+         * to L1 must be filled, and in the other all of the values related to
+         * L5 must be filled.
          *
          * If the data is available, gnssClockFlags must contain
          * HAS_CARRIER_FREQUENCY.