Updates EventPayload documentation for (Geomag)-RV

The former HAL documentation incorrectly documented (Geomag)-RV
sensor data as Vec4, although an accuracy field is expected according
to the Android sensor docs. Former default HAL implementation has set
the accuracy value to zero, preventing apps from getting this value.

This change guides OEMs to use the Vec4 + accuracy when converting
(Geomag)-RV sensor events. The default HAL implementation passes
this extra data now (ag/5224072), but clients of the former
implementation will still get the data it needs if assuming
the Vec4 sensor data format.

Bug: 116874058
Test: Compile only
Change-Id: I6a5c8a48dd372c3d4682ed5329f7f87862746cb9