- Create a glossary entry for experimental features.
- Have the man page experimental feature notice link `nix-commmand`.
(Eventually this should be programmed, based on whether the command is
experimental, and if so what experimental feature does it depend on.)
- Document which installables depend on which experimental features.
I tried to use the same style (bold warning and block quote) that the
top of the man page uses.
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
