Options for RR

[firelizzard18]

This is a heads up and/or discussion more than a specific request. go-delve/delve#3726 will be adding two environment variables to delve to enable passing flags to rr replay and rr record. I believe I can already set environment variables for delve, so no change is needed, but for the sake of visibility it seems worth documenting. Though now that I look for it, I can't find any documentation on using RR with vscode-go beyond it being a possible backend value, so maybe this issue should be "Document debugging with RR". I also thought it could be nice to have launch options (e.g. "traceReplayFlags") that were passed to the new environment variables, but I'm fine without that.