Merge #13418: Docs: More precise explanation of parameter onlynet

Summary:
2454a8558a Docs: More precise explanation of parameter "onlynet" (wodry)

Pull request description:

See issue #13378

Tree-SHA512: d51e81e4ef7fe084c3c9accf3433760fb34c02d149bb2d7006545fecdf68ecd4a5c5bfd0585fd5caff2f034eb96c7da80e85cb04c0ff3edb4c9c65ab56eb2847

Backport of Core PR13418

Test Plan:

ninja
src/bitcoind --help

Verify change in help text.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6672