A hostname is called fully qualified when a domain name is appended to it.
servername is not fully qualified, servername.domainname.org is.
When the command
hostname is used to return the hostname of a system,
hostname -f is used to return the fully qualified hostname.
If you run this command on your system and it return nothing, then you may expect this warning in Apache and in some other programs too.
In this case, if you really want to cure to problem, you will have to define
/etc/hosts accordingly :
<your ip> servername.domainname servername
Example, if my IP is 10.1.1.1, my domain example.com and my hostname host1, a minimal
/etc/hosts file would be :
127.0.0.1 localhost.localdomain localhost
10.1.1.1 host1.example.com host1
After this, the command
hostname will return host1 and
hostname -f returns host1.example.com.
By doing so, no need to tell your Apache that your hostname is localhost. No need to modify the main Apache configuration file, so the next time Apache will be updated, you won't loose your setting or have the new config file not merged into the existing one.