x/net/ipv4: better wording for TODOs
x/net/ipv4: better wording for TODOs

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/167450043

golang/net,veliamsli/net,flowlo/go-net,DramaFever/golang.org-x-net,muzining/net,dafyddcrosby/net,gomini/net,cesanta/goxnet,cmars/tools,kiip/net,mckinney-subgraph/net,albertjin/golang-net,chenzlabs/net,w4ngyi/net,muzining/net,cesanta/goxnet,mckinney-subgraph/net,raboof/net,zabawaba99/net,dafyddcrosby/net,zabawaba99/net,albertjin/golang-net,gomini/net,DramaFever/golang.org-x-net,subuk/net,raboof/net,yangxianjun/net,cmars/tools,cmars/tools,kiip/net,flowlo/go-net,cmars/tools,foreversmart/net,veliamsli/net,subuk/net,chenzlabs/net,w4ngyi/net,yangxianjun/net,foreversmart/net