Document the diff format

[tjvr]

The diff format is similar to the one used by json-diff. It looks something like this (IIRC):

• [' '] -- the item is unchanged
• ['+', <object>] -- the item has been added
• ['-', <object>] -- item was removed
• ['~', <diff>] -- item was modified. The diff structure is recursive, so diff might be an object or an array

{__old: <object>, __new: <object>} is the diff format for 'replace this object with another'.