My previous attempt to document this wasn't great.
Reworked the text and diagram to better illustrate what's going on.

Still using 'interleaved-by-lane' terminology for now.

Bug: b/126330097
Change-Id: Ia6031f06f9eab0f1f05d80151d9a89de5525b5eb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26529Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <headlessclayton@gmail.com>