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

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - skimj (skimj) 

Attached to Project - OpenWrt/LEDE Project
Summary - add documentation for dnsmasq option conf-dir
Task Type - Bug Report
Category - Documentation
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - The dnsmasq option for conf-dir is implemented but not documented (website or otherwise) Something like this:
| 'confdir' | string | '/tmp/dnsmasq.d' | '--conf-dir' | Read all the files in the given directory as configuration files.

The dnsmasq documentation seems to be a mess and I'm not even sure where specifically this option should be included.

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