dt-bindings: sensor: lsm9ds1: Fix misleading descriptions

The LSM9DS1 bindings were described as a full 9-axis IMU, while each
binding only covers one part of the device. This also made the
"Supported features" section of the generated docs misleading, since it
only shows the first sentence.

Update the descriptions to explicitly state whether they cover the
accelerometer + gyroscope or the magnetometer.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>