Minor doc update, clarifying intent of 'CInv'
Test Plan: comment only, code untouched
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2186