Fix the pages in the manual for Lix

This doesn't comprehensively fix everything outdated in the manual, or
make the manual greatly better, but it does note down where at least
jade noticed it was wrong, and it does fix all the instances of
referencing Nix to conform to the style guide to the best of our
ability.

A lot of things have been commented out for being wrong, and there are
three types of FIXME introduced:

- FIXME(Lix): generically Lix needs to fix it
- FIXME(Qyriad): re https://git.lix.systems/lix-project/lix/issues/215
- FIXME(meson): docs got outdated by meson changes and need rewriting

I did fix a bunch of it that I could, but there could certainly be
mistakes and this is definitely just an incremental improvement.

Fixes: https://git.lix.systems/lix-project/lix/issues/266
Change-Id: I5993c4603d7f026a887089fce77db08394362135

src/nix/upgrade-nix.md

@@ -16,8 +16,8 @@ R""(
 
 # Description
 
-This command upgrades Nix to the stable version declared in Nixpkgs.
-This stable version is defined in [nix-fallback-paths.nix](https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix)
+This command upgrades Lix to the latest stable version.
+This stable version is defined in [the Lix manifest](https://releases.lix.systems/manifest.nix)
 and updated manually. It may not always be the latest tagged release.
 
 By default, it locates the directory containing the `nix` binary in the `$PATH`