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

[LEDE Bugs]

The following task has a new comment added:

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

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It's a wiki, go ahead and change/improve it!
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1365#comment4352