doc: describe onlynet option in doc/tor.md

Summary:

as per http://www.erisian.com.au/bitcoin-core-dev/log-2019-04-11.html#l-102.

The typo in dependencies.md fixed by this PR was already corrected on our side.

Backport of Core PR15792
Depends on D8001

Test Plan: Proof-reading

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8002