relnotes: Remove references to documentation for !-RELEASE

The release notes erroneously reference documentation that is not
provided with either -CURRENT or -STABLE branches.

If they ever existed, it's been a long time since they were removed, as
a check on the ftp-archive reveals no existence of it as far back as
6-STABLE in and around 2005.

PR: 252980 255242
Reviewed by: gjb (releng)
Differential Revision: https://reviews.freebsd.org/D33073