Use consistent terminology to refer to a section in CAR file

[masih]

The term "Section" is used to describe the repeated <varint><cid><block> format that collectively make up the Data portion of a CAR file.

Use terminology consistent with docs in code to avoid confusion.
Example inconsistent terminology, where the term "block" is used to refer to a "Section":

go-car/v2/blockstore/readonly.go

Lines 184 to 187 in c4b9dc8

	func (b *ReadOnly) readBlock(idx int64) (cid.Cid, []byte, error) {
	bcid, data, err := util.ReadNode(internalio.NewOffsetReadSeeker(b.backing, idx), b.opts.ZeroLengthSectionAsEOF)
	return bcid, data, err
	}

[ribasushi]

I'd recommend using frame. Section is too broad / will conflict with the concept of synthetic cars in the future ( car files that are copy-pasted one after the other with all headers removed )

[masih]

Frame is good too; really, consistent terminology is the main thing for me :)

[rvagg]

I'm fine with "Frame" but it's going to require spec updating - someone needs to do that and you're welcome to (and I'd +1), but if that doesn't get done then "Section" is what we have.