docs: Describe / as a composefs mount in bootc-sysroot(7)

The text predates composefs and still says / is a bind mount of the
deployment directory. With composefs enabled, which the docs
recommend and the composefs backend always uses, / is a composefs
mount instead, and its mount source is no longer the root block
device. That matters for tools that look at / to find the disk.

Assisted-by: AI
Signed-off-by: Joseph Marrero Corchado <jmarrero@redhat.com>

Author: jmarrero

docs/src/bootc-sysroot.7.md

@@ -19,11 +19,13 @@ In the future, this functionality will be exposed and used by `bootc`.
 ## /sysroot mount
 
 When booted, the physical root will be available at `/sysroot` as a
-read-only mount point and the logical root `/` will be a bind mount
-pointing to a deployment directory under `/sysroot/ostree`.  This is a
-key aspect of how `bootc upgrade` operates: it fetches the updated
-container image and writes the base image files (using OSTree storage
-to `/sysroot/ostree/repo`).
+read-only mount point. With composefs enabled (as recommended in
+[filesystem](bootc-filesystem.7.md), and always with the composefs
+backend), the logical root `/` is a read-only composefs mount of the
+deployed image; otherwise it is a bind mount of a deployment directory
+under `/sysroot/ostree`.  This is a key aspect of how `bootc upgrade`
+operates: it fetches the updated container image and writes the base
+image files (using OSTree storage to `/sysroot/ostree/repo`).
 
 Beyond that and debugging/introspection, there are few use cases for tooling to
 operate on the physical root.