config: Clarify mounts[].source relative path anchor

Borrowing language from root.path for absolute/relative.

I think we also need to talk about 'mounts' being in the container
mount namespace (after creating it with the runtime mount namespace as
a seed) and before the pivot into root.path.  But Michael and/or
Mrunal are going to file follow-up work to address that.

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -61,6 +61,7 @@ For Windows, see [mountvol][mountvol] and [SetVolumeMountPoint][set-volume-mount
   * Windows: the type of file system on the volume, e.g. "ntfs".
   * Solaris: corresponds to "type" of the fs resource in [zonecfg(1M)][zonecfg.1m].
 * **`source`** (string, OPTIONAL) A device name, but can also be a directory name or a dummy.
+  Path values are either absolute or relative to the bundle.
   * Windows: the volume name that is the target of the mount point, \\?\Volume\{GUID}\ (on Windows source is called target).
   * Solaris: corresponds to "special" of the fs resource in [zonecfg(1M)][zonecfg.1m].
 * **`options`** (list of strings, OPTIONAL) Mount options of the filesystem to be used.