Corrections to classic format description

[Dave-Allured]

Classic format has three parts, not two.
Also minor edits for readability.

[WardF]

@ethanrd Ethan, any feedback on this, or (logistically) how it will impact the branch you're working on?

[ethanrd]

Hi @Dave-Allured, @WardF - This seems like a matter of interpretation or preference. The format itself doesn't change whether it is described as two parts (header and data) with the data then broken into two sections (non-record data and record data) or it is described as three parts.

So it doesn't feel like there's a strong need for this change. And since both the text and the BNF specification have historically used the two-part description I think a change should require a strong argument. Do you have a reason beyond preference?

[Dave-Allured]

@ethanrd, yes, this is a matter of preference. I do not have a strong argument.

However, the current function doc for nc__enddef includes a well written three-part description, not two. I think the user's guide should follow that example. In my experience, awareness of the three parts has been continuously helpful for data set design, performance issues, and related service questions. I rest my case.