doc: document use of Refs: for references

Standardise on Refs:

PR-URL: nodejs#10670
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

COLLABORATOR_GUIDE.md

@@ -120,6 +120,7 @@ information regarding the change process:
   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
   appropriate.
+- A `Refs:` line referencing a URL for any relevant background.
 - A `Reviewed-By: Name <email>` line for yourself and any
   other Collaborators who have reviewed the change.
   - Useful for @mentions / contact list if something goes wrong in the PR.

CONTRIBUTING.md

@@ -136,10 +136,13 @@ Check the output of `git log --oneline files_that_you_changed` to find out
 what subsystem (or subsystems) your changes touch.
 
 If your patch fixes an open issue, you can add a reference to it at the end
-of the log. Use the `Fixes:` prefix and the full issue URL. For example:
+of the log. Use the `Fixes:` prefix and the full issue URL. For other references
+use `Refs:`. For example:
 
 ```txt
 Fixes: https://github.com/nodejs/node/issues/1337
+Refs: http://eslint.org/docs/rules/space-in-parens.html
+Refs: https://github.com/nodejs/node/pull/3615
 ```
 
 ### Step 4: Rebase