Comment links to -STABLE release notes

As a side effect of moving the release notes from the src/ tree to
the doc/ tree, the -STABLE release notes have not been functional.
It does not seem feasible to reimplement them at this point, since
the conversion from XML to AsciiDoc.

PR: 259148
Submitted by: freebsd@frost _dot_ kiwi
Sponsored by: Rubicon Communications, LLC ("Netgate")