[MRG+1] tests+doc for subdomains in offsite middleware

[Digenis]

While explaining to a colleague how to restrict crawls to a specific vhost by specifying a subdomain
I noticed that the documentation doesn't demonstrate the subdomain behaviour clearly.
I then tried to point to the tests but they also lacked such an example.
As a test case it's redundant because the regex is actually tested on the TLD
so you may want me to amend the commit first.
I'd only consider it for the future possibility that someone tries to refactor the offsite middelware
and accidentally treats the top level and second level domains as a special case.

[codecov-io]

Current coverage is 83.25%

Merging #1721 into master will not affect coverage as of c3696e6

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[eliasdorneles]

Thanks @Digenis