docs: Add a man page for bootc-root-setup

On general principle.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

docs/src/SUMMARY.md

@@ -58,6 +58,7 @@
 
 - [bootc image](experimental-bootc-image.md)
 - [composefs backend](experimental-composefs.md)
+- [`man bootc-root-setup.service`](man/bootc-root-setup.service.5.md)
 - [fsck](experimental-fsck.md)
 - [install reset](experimental-install-reset.md)
 - [--progress-fd](experimental-progress-fd.md)

docs/src/man/bootc-root-setup.service.5.md

@@ -0,0 +1,73 @@
+# NAME
+
+bootc-root-setup.service
+
+# DESCRIPTION
+
+This service runs in the initramfs to set up the root filesystem when composefs is enabled.
+It is only activated when the `composefs` kernel command line parameter is present.
+
+The service performs the following operations:
+
+- Mounts the composefs image specified in the kernel command line
+- Sets up `/etc` and `/var` directories from the deployment state
+- Optionally configures transient overlays based on the configuration file
+- Prepares the root filesystem for switch-root
+
+This service runs after `sysroot.mount` and `ostree-prepare-root.service`, and before
+`initrd-root-fs.target`.
+
+# CONFIGURATION FILE
+
+The service reads an optional configuration file at `/usr/lib/composefs/setup-root-conf.toml`.
+If this file does not exist, default settings are used.
+
+**WARNING**: The configuration file format and composefs integration are experimental
+and subject to change.
+
+## Configuration Options
+
+The configuration file uses TOML format with the following sections:
+
+### `[root]`
+
+- `transient` (boolean): If true, mounts the root filesystem as a transient overlay.
+  This makes all changes to `/` ephemeral and lost on reboot. Default: false.
+
+### `[etc]`
+
+- `mount` (string): Mount type for `/etc`. Options: "none", "bind", "overlay", "transient".
+  Default: "bind".
+- `transient` (boolean): Shorthand for `mount = "transient"`. Default: false.
+
+### `[var]`
+
+- `mount` (string): Mount type for `/var`. Options: "none", "bind", "overlay", "transient".
+  Default: "bind".
+- `transient` (boolean): Shorthand for `mount = "transient"`. Default: false.
+
+## Example Configuration
+
+```toml
+[root]
+transient = false
+
+[etc]
+mount = "bind"
+
+[var]
+mount = "overlay"
+```
+
+# EXPERIMENTAL STATUS
+
+The composefs integration, including this service and its configuration file format,
+is experimental and subject to change.
+
+# SEE ALSO
+
+**bootc(8)**
+
+# VERSION
+
+<!-- VERSION PLACEHOLDER -->