docs: Mention the new mount API in the container interface doc

Let's mention that the new mount API may be used to establish new
mounts in a container without needing the /run/host/incoming directory.

(cherry picked from commit 74cc5e2041a2c32e1824b32316bd95f2c8a811f5)
(cherry picked from commit 65eff444c4fa7be5eb1be71c5d94ab8732167e11)
(cherry picked from commit 53d92de)
(cherry picked from commit c12ef9e)

docs/CONTAINER_INTERFACE.md

@@ -230,7 +230,9 @@ care should be taken to avoid naming conflicts. `systemd` (and in particular
    directory: it's used by code outside the container to insert mounts inside
    it only, and is mostly an internal vehicle to achieve this. Other container
    managers that want to implement similar functionality might consider using
-   the same directory.
+   the same directory. Alternatively, the new mount API may be used by the
+   container manager to establish new mounts in the container without the need
+   for the `/run/host/incoming/` directory.
 
 2. The `/run/host/inaccessible/` directory may be set up by the container
    manager to include six file nodes: `reg`, `dir`, `fifo`, `sock`, `chr`,