doc: note that PR-URL should be first metadata

Convention has coalesced around putting the PR-URL as the first metadata
item in pull requests, so much so that collaborators are now flagging
other collaborators' commit messages when they don't do that. So let's
make it official and document that the PR-URL should be first.

Refs: nodejs/build#325
Refs: nodejs#6385

COLLABORATOR_GUIDE.md

@@ -91,11 +91,12 @@ The CTC should serve as the final arbiter where required.
 Always modify the original commit message to include additional meta
 information regarding the change process:
 
-- A `Reviewed-By: Name <email>` line for yourself and any
-  other Collaborators who have reviewed the change.
 - A `PR-URL:` line that references the *full* GitHub URL of the original
   pull request being merged so it's easy to trace a commit back to the
-  conversation that led up to that change.
+  conversation that led up to that change. Please be sure that the `PR-URL`
+  line appears first, above the `Reviewed-By` lines and any `Fixes` lines.
+- A `Reviewed-By: Name <email>` line for yourself and any
+  other Collaborators who have reviewed the change.
 - A `Fixes: X` line, where _X_ either includes the *full* GitHub URL
   for an issue, and/or the hash and commit message if the commit fixes
   a bug in a previous commit. Multiple `Fixes:` lines may be added if

doc/onboarding.md

@@ -164,11 +164,11 @@ Landing a PR
   * commits should follow `subsystem[,subsystem]: small description\n\nbig description\n\n<metadata>`
   * first line 50 columns, all others 72
   * add metadata:
+    * `PR-URL: <full-pr-url>` (this line should always be the first metadata line!)
     * `Fixes: <full-issue-url>`
     * `Reviewed-By: human <email>`
       * Easiest to use `git log` then do a search
       * (`/Name` + `enter` (+ `n` as much as you need to) in vim)
-    * `PR-URL: <full-pr-url>`
 * `git push upstream master`
     * close the original PR with "Landed in `<commit hash>`".
 
@@ -180,7 +180,7 @@ Landing a PR
   * Collaborators in alphabetical order by username
   * Label your pull request with the `doc` subsystem label
   * If you would like to run CI on your PR, feel free to
-  * Make sure to added the `PR-URL: <full-pr-url>`!
+  * Make sure to add the `PR-URL: <full-pr-url>`!
 
 
 ### final notes: