Merge #16188: net: Document what happens to getdata of unknown type

Summary:
dddd9270f85bd2e71fd281a0c6b4053e02fce93c net: Document what happens to getdata of unknonw type (MarcoFalke)

Pull request description:

Any getdata of unknown type will never be processed and blocks all future messages from a peer. This isn't obviously clear from reading the code, so document it.

Top commit has no ACKs.

Tree-SHA512: 4f8e43bbe6534242facfcfffae28b7a6aa2d228841fa2146a87d494e69f614b0da23cf7a5f3d4367358a7c1981fe2ec196a21c437ae1653f1c7e0351be22598a

Backport of Core PR16188

Adjusted the second comment to actually make sense.

Test Plan:

ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6625