luci-mod-network: update dhcp and dns labels and descriptions

luci-mod-network: update dhcp and dns labels and descriptions

Update labels and descriptions for DHCP and DNS settings.
Add missing descriptions and clarify the existing ones.
Group settings by tab and avoid redundant abbreviations.
Consolidate terminology according to the dnsmasq documentation:
https://thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html
Fix spelling according to the previously adopted policy:
https://github.com/openwrt/luci/commit/88b9d843882cf52a6acf4d08a878fd005120edd4

Signed-off-by: Vladislav Grigoryev <vg.aetera@gmail.com>