[docs] Added all supported paramenters of WireGuard backend #297

[pandafy]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #297

[coveralls]

coverage: 99.497%. remained the same
when pulling 6f8d8bb on issues/297-wiregaurd-docs
into 9b2c214 on master.

[nemesifier]

I would change the description here to just "interface MTU".

I was confused by the default value here because I remember the default MTU being 1420, but on IPv6 is 1280.

I just realized this is not a great situation. I am thinking about the easiest acceptable solution here..

We could set the default to 1420 both here and in the schema, and add a help text (description property in the schema) which says: "Interface MTU, set to 1280 if using IPv6."

Alternatively, we could remove the default and state in the description to use 1420 or 1280 whether using ipv4 or ipv6, however, I using IPv4 is the norm, so the first solution is probably going to be the most comfortable for most users.

What do you think?

[pandafy]

I like the first solution more. I have made changes accordingly.