Explain the term "schema" in definition section

[agnat]

The term schema pops up first in the format section of the specification. Since it is such a central term, I feel it should be explained in the definition section.

[MikeRalphson]

Belated thanks for bringing this up. A PR with candidate wording would be gratefully received.

[ralfhandl]

Fixed in 3.0.4 and 3.1.1: @handrews can we close this?

[handrews]

@ralfhandl yes, I thought I'd gone through and closed everything - I'll double-check.