dhcp.cgi: Adjust legend entries to make clear they are legends and not messages

dhcp.cgi: Adjust legend entries to make clear they are legends and not messages

- A new IPFire user on the forum saw the orange and red coloured blocks in the legend
   section and believed that they were messages about problems that had been created with
   the fixed leases.
- This change puts a small block with seperate explanatory text for both the orange and
   red coloured blocks.
- This change will also be applied to the wiki in a much clearer way

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>