Review user documentation of 'lazy' concept (5 man days)

[pp-mo]

See : docs/iris/src/developers_guide/dask_interface.rst.
If that's the developer view, what does a user need to know?

Acceptance:

• updated above (developer) document
  • up-to-date with new internal API
  • clear distinction between what is internal to cube.py, and what is "public" for use elsewhere
  • details of fill-value support
  • clarification of cube metadata meaning and usage
• new user documentation explaining:
  • basics of how Iris works with Dask
  • the handling of fill-values

[DPeterK]

Replaced by #2509