editMeta: tighten contract via docs (#5714)

Spigot-API-Patches/ItemStack-editMeta.patch

@@ -15,6 +15,13 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +    // Paper start
 +    /**
 +     * Edits the {@link ItemMeta} of this stack.
++     * <p>
++     * The {@link java.util.function.Consumer} must only interact
++     * with this stack's {@link ItemMeta} through the provided {@link ItemMeta} instance.
++     * Calling this method or any other meta-related method of the {@link ItemStack} class
++     * (such as {@link #getItemMeta()}, {@link #addItemFlags(ItemFlag...)}, {@link #lore()}, etc.)
++     * from inside the consumer is disallowed and will produce undefined results or exceptions.
++     * </p>
 +     *
 +     * @param consumer the meta consumer
 +     * @return {@code true} if the edit was successful, {@code false} otherwise