[FS#1365] add documentation for dnsmasq option conf-dir

[LEDE Bugs]

The following task is now closed:

FS#1365 - add documentation for dnsmasq option conf-dir
User who did this - Mathias Kresin (mkresin)

Reason for closing: Deferred
Additional comments about closing: OpenWrt depends on user contribution, go ahead and improve the existing documentation. So far it doesn't really look like a "bug".

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1365