Generate the nix.conf docs from the source code

This means we don't have two (divergent) sets of option descriptions
anymore.

doc/manual/src/command-ref/conf-file-prefix.md

@@ -0,0 +1,39 @@
+Title: nix.conf
+
+# Name
+
+`nix.conf` - Nix configuration file
+
+# Description
+
+By default Nix reads settings from the following places:
+
+  - The system-wide configuration file `sysconfdir/nix/nix.conf` (i.e.
+    `/etc/nix/nix.conf` on most systems), or `$NIX_CONF_DIR/nix.conf` if
+    `NIX_CONF_DIR` is set. Values loaded in this file are not forwarded
+    to the Nix daemon. The client assumes that the daemon has already
+    loaded them.
+
+  - If `NIX_USER_CONF_FILES` is set, then each path separated by `:`
+    will be loaded in reverse order.
+
+    Otherwise it will look for `nix/nix.conf` files in `XDG_CONFIG_DIRS`
+    and `XDG_CONFIG_HOME`. If these are unset, it will look in
+    `$HOME/.config/nix.conf`.
+
+The configuration files consist of `name =
+value` pairs, one per line. Other files can be included with a line like
+`include
+path`, where *path* is interpreted relative to the current conf file and
+a missing file is an error unless `!include` is used instead. Comments
+start with a `#` character. Here is an example configuration file:
+
+    keep-outputs = true       # Nice for developers
+    keep-derivations = true   # Idem
+
+You can override settings on the command line using the `--option` flag,
+e.g. `--option keep-outputs
+false`.
+
+The following settings are currently available:
+