The maxWriteBatchSize was introduced in 2.6 (
SERVER-12275) and specifies the maximum number of write operations that may appear in a single insert, update, or delete command.
This value was originally 1000 when first introduced but has increased over time. 3.6 reports a limit of 100,000. I don't think we need to document the exact version returned by each server version. It would be sufficient to simply mention what the field is. Drivers are instructed to respect this (along with maxBsonObjectSize and maxMessageSizeBytes) when splitting bulk write operations into commands to send over the wire.