docs: fix [Build] Volume= to not reference named volumes

The [Build] Volume= documentation incorrectly described support for
SOURCE-VOLUME (named volumes) and .volume Quadlet unit references.
These features are only available in [Container] Volume=, not in
[Build] Volume=, because podman build --volume only supports host
directory bind mounts.

Simplify the description to match the actual podman build --volume
behavior and add a note clarifying the difference from [Container]
Volume=.

Fixes #25514

Signed-off-by: Kit Dallege <xaum.io@gmail.com>

Author: kovan

docs/source/markdown/podman-systemd.unit.5.md

@@ -2053,15 +2053,13 @@ This is equivalent to the `--variant` option of `podman build`.
 
 ### `Volume=`
 
-Mount a volume to containers when executing RUN instructions during the build. This is equivalent to
-the `--volume` option of `podman build`, and generally has the form
-`[[SOURCE-VOLUME|HOST-DIR:]CONTAINER-DIR[:OPTIONS]]`.
+Mount a host directory into containers when executing RUN instructions during the build. This is equivalent to
+the `--volume` option of `podman build`, and has the form
+`[HOST-DIR:]CONTAINER-DIR[:OPTIONS]`.
 
-If `SOURCE-VOLUME` starts with `.`, Quadlet resolves the path relative to the location of the unit file.
-
-Special case:
+If `HOST-DIR` starts with `.`, Quadlet resolves the path relative to the location of the unit file.
 
-* If `SOURCE-VOLUME` ends with `.volume`, Quadlet will look for the corresponding `.volume` Quadlet unit. If found, Quadlet will use the name of the Volume set in the Unit, otherwise, `systemd-$name` is used. The generated systemd service contains a dependency on the service unit generated for that `.volume` unit, or on `$name-volume.service` if the `.volume` unit is not found. Note: the corresponding `.volume` file must exist.
+Note: unlike the `[Container]` section's `Volume=`, this option does not support named volumes or `.volume` Quadlet unit references, because `podman build --volume` only supports host directory bind mounts.
 
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