Merge changes I5566a985,I88cf53d3 into main

* changes:
  Support relative and `~/` paths in config settings
  Thread `ApplyConfigOptions` through config parsing

doc/manual/rl-next/relative-and-tilde-paths-in-config.md

@@ -0,0 +1,30 @@
+---
+synopsis: Relative and tilde paths in configuration
+issues: [fj#482]
+cls: [1851, 1863, 1864]
+category: Features
+credits: [9999years]
+---
+
+[Configuration settings](@docroot@/command-ref/conf-file.md) can now refer to
+files with paths relative to the file they're written in or relative to your
+home directory (with `~/`).
+
+This makes settings like
+[`repl-overlays`](@docroot@/command-ref/conf-file.md#conf-repl-overlays) and
+[`secret-key-files`](@docroot@/command-ref/conf-file.md#conf-repl-overlays)
+much easier to set, especially if you'd like to refer to files in an existing
+dotfiles repo cloned into your home directory.
+
+If you put `repl-overlays = repl.nix` in your `~/.config/nix/nix.conf`, it'll
+load `~/.config/nix/repl.nix`. Similarly, you can set `repl-overlays =
+~/.dotfiles/repl.nix` to load a file relative to your home directory.
+
+Configuration files can also
+[`include`](@docroot@/command-ref/conf-file.md#file-format) paths relative to
+your home directory.
+
+Only user configuration files (like `$XDG_CONFIG_HOME/nix/nix.conf` or the
+files listed in `$NIX_USER_CONF_FILES`) can use tilde paths relative to your
+home directory. Configuration listed in the `$NIX_CONFIG` environment variable
+may not use relative paths.