Add text to algorithmic components sections in LDM-151

While John Swinbank has commented that the outlines are probably good enough for planning work (and I thnk that's broadly true), the lack of text in the algorithmic components section did occasionally lead to some misunderstandings in Robert Lupton's first review pass, so I think I should flesh that out with text sooner rather than later.

In this issue, I'll stick to sections that no one else has added text for, but eventually I'll also need to work with Simon Krughoff and perhaps others to ensure that section has a consistent level of detail and focus.