net: improve nLastBlockTime and nLastTXTime documentation

Summary:

[...] help alleviate confusion around nLastBlockTime and nLastTXTime,
now also provided by the JSON-RPC API as last_block and last_transaction
in getpeerinfo output.

Backport of core#19857.

Depends on D9331.

Test Plan: Read the comments.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9332