ssh: comment deprecated option handling for retired local patches

Older versions of FreeBSD included the HPN patch set and provided
client-side VersionAddendum. Both of these changes have been retired
but we've retained the option parsing for backwards compatibility to
avoid breaking upgrades. Add comment references to the relevant
commits.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit a93cbba2deed209a30da1ecffa637bf5c61b7641)