doc: update -onlynet help in src/init.cpp
doc: update -onlynet help in src/init.cpp

Summary:
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20757 | core#20757]] [1/2]
https://github.com/bitcoin/bitcoin/pull/20757/commits/784a278e872ea498dbc313a5a41a8d9f912adb7c

Test Plan: `ninja && src/bitcoind -help`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11030

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