Add documentation for ipv6_neighbor_router_advertisement.rb see https…

[basicallyabidoof]

…://github.com//issues/12389

Tell us what this change does. If you're fixing a bug, please mention
the github issue number.

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Verification

List the steps needed to make sure this thing works

• Start msfconsole
• use exploit/windows/smb/ms08_067_netapi
• ...
• Verify the thing does what it should
• Verify the thing does not do what it should not
• Document the thing and how it works (Example)

If you are opening a PR for a new module that exploits a specific piece of hardware or requires a complex or hard-to-find testing environment, we recommend that you send us a demo of your module executing correctly. Seeing your module in action will help us review your PR faster!

Specific Hardware Examples:

• Switches
• Routers
• IP Cameras
• IoT devices

Complex Software Examples:

• Expensive proprietary software
• Software with an extensive installation process
• Software that requires exploit testing across multiple significantly different versions
• Software without an English language UI

We will also accept demonstrations of successful module execution even if your module doesn't meet the above conditions. It's not a necessity, but it may help us land your module faster!

Demonstration of successful module execution can take the form of a packet capture (pcap) or a screen recording. You can send pcaps and recordings to msfdev@metasploit.com. Please include a CVE number in the subject header (if applicable), and a link to your PR in the email body.
If you wish to sanitize your pcap, please see the wiki.