config.md: allow empty mappings for [r]idmap

crun currently allows to specify an empty mapping for [r]idmap, and to
default to the mappings specified for the container user namespace.

Change the specifications to allow such behavior.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

config.md

@@ -151,8 +151,8 @@ Runtimes MUST/SHOULD/MAY implement the following option strings for Linux:
  `sync`           | MUST        | [^1]
  `tmpcopyup`      | MAY         | copy up the contents to a tmpfs
  `unbindable`     | MUST        | Bind mount propagation [^2]
- `idmap`          | SHOULD      | Indicates that the mount has `uidMappings` and `gidMappings` specified. This option SHOULD NOT be passed to the underlying [`mount(2)`][mount.2] call. If supported, the runtime MUST return an error if this option is provided and either of `uidMappings` or `gidMappings` are empty or not present.
- `ridmap`         | SHOULD      | Indicates that the mount has `uidMappings` and `gidMappings` specified, and the mapping is applied recursively [^3]. This option SHOULD NOT be passed to the underlying [`mount(2)`][mount.2] call. If supported, the runtime MUST return an error if this option is provided and either of `uidMappings` or `gidMappings` are empty or not present.
+ `idmap`          | SHOULD      | Indicates that the mount MUST have an idmapping applied. This option SHOULD NOT be passed to the underlying [`mount(2)`][mount.2] call. If `uidMappings` or `gidMappings` are not specified, the runtime MAY use the container's user namespace mapping.  If there are no `uidMappings` and `gidMappings` specified and the container isn't using user namespaces, an [error MUST be returned](runtime.md#errors). This SHOULD be implemented using [`mount_setattr(MOUNT_ATTR_IDMAP)`][mount_setattr.2], available since Linux 5.12.
+ `ridmap`         | SHOULD      | Indicates that the mount MUST have an idmapping applied, and the mapping is applied recursively [^3]. This option SHOULD NOT be passed to the underlying [`mount(2)`][mount.2] call. If `uidMappings` or `gidMappings` are not specified, the runtime MAY use the container's user namespace mapping.  If there are no `uidMappings` and `gidMappings` specified and the container isn't using user namespaces, an [error MUST be returned](runtime.md#errors). This SHOULD be implemented using [`mount_setattr(MOUNT_ATTR_IDMAP)`][mount_setattr.2], available since Linux 5.12.
 
 [^1]: Corresponds to [`mount(8)` (filesystem-independent)][mount.8-filesystem-independent].
 [^2]: Corresponds to [bind mounts and shared subtrees][mount-bind].