doc: add info on post-publishing ARM6 builds

ARM6 takes longer, more often fails builds than other
platforms.  Add instructions for how to recover in this
case.

PR-URL: https://github.com/nodejs/node/pull/17455


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>