Bluetooth: host: Don't auto-update device name and document behavior

Don't auto-update the device name in the advertising data as that
would clear any other scan-response data set by the application.
Document the behavior of the advertise with device name option
and the need to update data by the application to make sure
advertising data does not get cleared.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>