Remove bulk spec and clarify a few sections

The original write command spec had lots of text about how to do batch
operations.  This has been superseded by the bulk API spec so I've
removed it for consistency and clarity.

I renamed/reordered the response fields section for clarity.

I also amended some other paragraphs that I thought could be confusing
or misleading.
Merge pull request #6 from dagolden/topic/clean-up-write-commands

Remove bulk spec and clarify a few sections