Bluetooth: GATT: Document settings_load and service register requirement

Document that all services that should be included in the initial
database hash should be registered before calling settings load. All
services added after settings_load will trigger a new hash calculation
and a new value stored. This would result in the database hash always
being rewritten.

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