doc: add suggestion to use --3way

The CI seems to do a 3way merge so it is possible
that even though the CI passed, the existing git am command
may fail.  Add text to suggest how to handle this
by adding the --3way option.

PR-URL: #12510
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: mhdawson

COLLABORATOR_GUIDE.md

@@ -372,6 +372,18 @@ Apply external patches
 $ curl -L https://github.com/nodejs/node/pull/xxx.patch | git am --whitespace=fix
 ```
 
+If the merge fails even though recent CI runs were successful, then a 3-way merge may
+be required.  In this case try:
+
+```text
+$ git am --abort
+$ curl -L https://github.com/nodejs/node/pull/xxx.patch | git am -3 --whitespace=fix
+```
+If the 3-way merge succeeds you can proceed, but make sure to check the changes
+against the original PR carefully and build/test on at least one platform
+before landing. If the 3-way merge fails, then it is most likely that a conflicting
+PR has landed since the CI run and you will have to ask the author to rebase.
+
 Check and re-review the changes
 
 ```text

doc/onboarding-extras.md

@@ -103,16 +103,6 @@ to update from nodejs/node:
 * `git merge --ff-only upstream/master` (or `REMOTENAME/BRANCH`)
 
 
-## If `git am` fails
-
-* if `git am` fails – use `git am --abort`
-  * this usually means the PR needs updated
-  * prefer to make the originating user update the code, since they have it fresh in mind
-* first, reattempt with `git am -3` (3-way merge)`
-* if `-3` still fails, and you need to get it merged:
-  * `git fetch upstream pull/N/head:pr-N && git checkout pr-N && git rebase master`
-
-
 ## best practices
 
 * commit often, out to your github fork (origin), open a PR