Documentation: devicetree: changesets do locking on their own meanwhile

Since commit 18322377 ("drivers/of: Export OF changeset functions"),
the mentioned functions do all necessary locking.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Fixes: 18322377 ("drivers/of: Export OF changeset functions")
Signed-off-by: Rob Herring <robh@kernel.org>