doc: update maintaining ICU guide

Do not ask to separate the changes in deps/icu-small and in the
LICENSE file.
Do not ask to remove the whitespace changes. The way commits are
landed now doesn't eliminate them anymore.

PR-URL: https://github.com/nodejs/node/pull/40658


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>