…c#24110)

Documentation for the `internal_networks` option of the Netflow input and module was missing.
Also the module's manifest did not declare the option so if it was not set in the module config
it would cause an error.

I did not see where a default was set for the netflow input's internal_networks option so I set that
to `private` to keep the old behavior before this was configurable.

Fixes elastic#24094

(cherry picked from commit 3ca53aa)