doc: clarify "Reviewed-By" iff "LGTM"

As per conversation with @Trott, make it clear that Reviewed-By lines
should only be added for collaborators who've actually put a LGTM on the
PR.

PR-URL: #7183
Reviewed-By: Myles Borins <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: JacksonTian - Jackson Tian <[email protected]>

Author: bengl

doc/onboarding.md

@@ -165,6 +165,7 @@ Landing a PR
     * `Reviewed-By: human <email>`
       * Easiest to use `git log` then do a search
       * (`/Name` + `enter` (+ `n` as much as you need to) in vim)
+      * Only include collaborators who have commented "LGTM"
     * `PR-URL: <full-pr-url>`
 * `git push upstream master`
     * close the original PR with "Landed in `<commit hash>`".