release: Use "debug info" in dist set descriptions

"Kernel (Debugging)" could suggest that this is a debugging kernel --
an alternative to a "Kernel (Standard)". These dist sets contain the
standalone debug info files for the kernel, base system, and other
components. Change the MANIFEST descriptions (used by bsdinstall's
dist selection menu) to use "debug info".

Reviewed by: brooks
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50254