Improve documentation by commendign to use try/catch

[buguibu]

I've been dealing with an invisible error in this code

RLMRealm *realm = [RLMRealm defaultRealm];
[realm beginWriteTransaction];
[self createOrUpdateInRealm:realm withJSONDictionary:serverResponse];
[realm commitWriteTransaction];

until i used try/catch for receive the "Invalid value" exception like this:

RLMRealm *realm = [RLMRealm defaultRealm];
[realm beginWriteTransaction];
@try
{
    [self createOrUpdateInRealm:realm withJSONDictionary:serverResponse];
}
@catch(NSException *exception)
{
    NSLog(@"Couldn't import due to exception: %@", exception.description);
}
@finally
{
    [realm commitWriteTransaction];
}

Maybe it has been due to, for some reason, that during testing some RLMExceptions do not raise, i don't know, but i'll asume this as a best practice.