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: https://github.com/nodejs/node/pull/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>