release: disable the 'reldoc' target after the ASCIIDoctor switch

The 'reldoc' target includes release-related documentation on
installation medium. Since the switch from XML to ASCIIDoctor,
the file locations have moved, and it will take some time to sort
out how this target should work now.

Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 7b1d1a1658ffb69eff93afc713f9e88ed8b20eac)