Merge #15078: rpc: Document bytessent_per_msg and bytesrecv_per_msg

Summary:
fab3f1467823c854cc88bc5a7e087263866ed87e rpc: Document bytessent_per_msg and bytesrecv_per_msg (MarcoFalke)

Pull request description:

Tree-SHA512: 11af7502933b3dae203d90e36b35019e0ebe67ee09aa77360a27547487bd2b6bcaa18c5f60bc21966291d2ccf44dfedebf40103c4db70a359400f535a66abb23

Backport of Core PR15078

Test Plan:

ninja
ninja check
ninja check-functional
src/bitcoind
src/bitcoin-cli help getpeerinfo

Verify changes in help text.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6472