[doc] mention whitelist is inbound, and applies to blocksonly
[doc] mention whitelist is inbound, and applies to blocksonly

Summary:
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/16555 | PR16555]]
Depends on D7975

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

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7976

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