Document chained batch

[vweevers]

We can copy some stuff from abstract-leveldown#chainedbatch.

In addition, make sure the sync option is documented in the right place (for both array batch and chained batch). See https://github.com/Level/leveldown/pull/540/files#r240022189.

[ralphtheninja]

Adding this to 5.0.0-rc1

[vweevers]

Also change this part of the batch() docs, no longer true:

Any entries where the type is 'put' that have a 'value' of undefined, null, [], '' or Buffer.alloc(0) will be stored as a zero-length character array and therefore be fetched during reads as either '' or Buffer.alloc(0) depending on how they are requested.

It throws now.