doc: add information about modules cache behavior

This publicly documents that adding native module names will resolve
the added entry instead of the native module.

It also updates the description why extensions are deprecated.

PR-URL: https://github.com/nodejs/node/pull/26971
Refs: https://github.com/nodejs/node/pull/25362


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>