Update backporting-to-release-lines.md?

[prog1dev]

Should there be info about why particular pull request should be backported or not? And in which branches?

Or maybe not in backporting-to-release-lines.md but somewhere else?

Ref #18357 (comment)

[trivikr]

cc @MylesBorins @nodejs/backporting

[gibfahn]

I think adding a couple of lines in the What should be backported makes sense.

What should be backported is basically "anything that a release team member wanted to backport but had merge conflicts when it was cherry-picked to a release line".

[apapirovski]

Going to close this out given the lack of movement. Feel free to reopen if you believe this is important but in that case... maybe consider a PR instead?