doc: clarify Backport-PR-URL metadata added automatically · nodejs/node@70b4d58

Original file line number	Diff line number	Diff line change
`@@ -75,8 +75,8 @@ line.`
`75`	`75`	5. Resolve conflicts using `git add` and `git cherry-pick --continue`.
`76`	`76`
`77`	`77`	`6. Leave the commit message as is. If you think it should be modified, comment`
`78`		- in the pull request. The `Backport-PR-URL` metadata does need to be added to
`79`		`-the commit, but this will be done later.`
	`78`	+ in the pull request. Do not manually add a `Backport-PR-URL` metadata entry.
	`79`	`+This is automatically added later.`
`80`	`80`
`81`	`81`	7. Verify that `make -j4 test` passes.
`82`	`82`