config: add reference to mount_setattr(2) for idmapped mounts

The idmapped mounts sections do not make any reference to how the
mapping should be implemented. Add a reference to MOUNT_ATTR_IDMAP since
that is what runtimes are expected to use.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

config.md

@@ -180,10 +180,12 @@ For POSIX platforms the `mounts` structure has the following fields:
     * Linux: filesystem types supported by the kernel as listed in */proc/filesystems* (e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs", "msdos", "proc", "nfs", "iso9660"). For bind mounts (when `options` include either `bind` or `rbind`), the type is a dummy, often "none" (not listed in */proc/filesystems*).
     * Solaris: corresponds to "type" of the fs resource in [zonecfg(1M)][zonecfg.1m].
 * **`uidMappings`** (array of type LinuxIDMapping, OPTIONAL) The mapping to convert UIDs from the source file system to the destination mount point.
+  This SHOULD be implemented using [`mount_setattr(MOUNT_ATTR_IDMAP)`][mount_setattr.2], available since Linux 5.12.
   If specified, the `options` field of the `mounts` structure SHOULD contain either `idmap` or `ridmap` to specify whether the mapping should be applied recursively for `rbind` mounts, as well as to ensure that older runtimes will not silently ignore this field.
   The format is the same as [user namespace mappings](config-linux.md#user-namespace-mappings).
   If specified, it MUST be specified along with `gidMappings`.
 * **`gidMappings`** (array of type LinuxIDMapping, OPTIONAL) The mapping to convert GIDs from the source file system to the destination mount point.
+  This SHOULD be implemented using [`mount_setattr(MOUNT_ATTR_IDMAP)`][mount_setattr.2], available since Linux 5.12.
   If specified, the `options` field of the `mounts` structure SHOULD contain either `idmap` or `ridmap` to specify whether the mapping should be applied recursively for `rbind` mounts, as well as to ensure that older runtimes will not silently ignore this field.
   For more details see `uidMappings`.
   If specified, it MUST be specified along with `uidMappings`.