docs/logically-bound: A bit more docs

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

docs/src/experimental-logically-bound-images.md

@@ -7,7 +7,18 @@ Tracking issue: <https://github.com/containers/bootc/issues/128>
 
 ## About logically bound images
 
-This experimental feature enables an association of container "app" images to a bootc system image. A similar approach to this is [physically bound](https://github.com/containers/bootc/issues/644) images. There are some trade-offs between the two approaches. Some benefits of logically bound images are:
+This experimental feature enables an association of container "app" images to a base bootc system image. Use cases for this include:
+
+- Logging (e.g. journald->remote log forwarder container)
+- Monitoring (e.g. [Prometheus node_exporter](https://github.com/prometheus/node_exporter))
+- Configuration management agents
+- Security agents
+
+These types of things are commonly not updated outside of the host, and there's a secondary important property: We *always* want them present and available on the host, possibly from very early on in the boot. In contrast with default usage of tools like `podman` or `docker`, images may be pulled dynamically *after* the boot starts; requiring functioning networking, etc. For example if the remote registry is unavailable temporarily, the host system may run for a longer period of time without log forwarding or monitoring, which can be very undesirable.
+
+Another simple way to say this is that logically bound images allow you to reference container images with the same confidence you can with `ExecStart=` in a systemd unit.
+
+The term "logically bound" was created to contrast with [physically bound](https://github.com/containers/bootc/issues/644) images. There are some trade-offs between the two approaches. Some benefits of logically bound images are:
 
 - The bootc system image can be updated without re-downloading the app image bits.
 - The app images can be updated without modifying the bootc system image, this would be especially useful for development work
@@ -42,7 +53,19 @@ GlobalArgs=--storage-opt=additionalimagestore=/usr/lib/bootc/storage
 
 Images are fetched using the global bootc pull secret by default (`/etc/ostree/auth.json`). It is not yet supported to configure `PullSecret` in these image definitions.
 
+## Garbage collection
+
+The bootc image store is owned by bootc; images will be garbage collected when they are no longer referenced
+by a file in `/usr/lib/bootc/bound-images.d`.
+
 ## Limitations
 
-- Currently, only the Image field of a `.image` or `.container` file is used to pull the image; per above not even `PullSecret=` is supported.
-- Images are not yet garbage collected
+The *only* field parsed and honored by bootc currently is the `Image` field of a `.image` or `.container` file.
+
+Other pull-relevant flags such as `PullSecret=` for example are not supported (see above).
+Another example unsupported flag is `Arch` (the default host architecture is always used).
+
+There is no mechanism to inject arbitrary arguments to the `podman pull` (or equivalent)
+invocation used by bootc. However, many properties used for container registry interaction
+can be configured via [containers-registries.conf](https://github.com/containers/image/blob/main/docs/containers-registries.conf.5.md)
+and apply to all commands operating on that image.