Merge pull request #491 from cgwalters/doc-bootloader

docs: Link to bootupd in prominent places

docs/src/bootc-images.md

@@ -77,3 +77,10 @@ container build tool supports producing OCI container images.
 You may find some references to this; it is no longer very useful
 and is not recommended.
 
+# The bootloader setup
+
+At the current time bootc relies on the [bootupd](https://github.com/coreos/bootupd/)
+project which handles bootloader installs and upgrades.  The invocation of
+`bootc install` will always run `bootupd` to perform installations.
+Additionally, `bootc upgrade` will currently not upgrade the bootloader;
+you must invoke `bootupctl update`.

docs/src/bootc-install.md

@@ -9,6 +9,12 @@ or virtualized), one needs a few key components:
 - kernel (and optionally initramfs)
 - root filesystem (xfs/ext4/btrfs etc.)
 
+The bootloader state is managed by the external [bootupd](https://github.com/coreos/bootupd/)
+project which abstracts over bootloader installs and upgrades.  The invocation of
+`bootc install` will always run `bootupd` to handle bootloader installation
+to the target disk.   The default expectation is that bootloader contents and install logic
+come from the container image in a `bootc` based system.
+
 The Linux kernel (and optionally initramfs) is embedded in the container image; the canonical location
 is `/usr/lib/modules/$kver/vmlinuz`, and the initramfs should be in `initramfs.img`
 in that directory.