improve IR processing docs

[tomjnixon]

• say exactly how the public data files were created (i.e. which snakemake recipes with which files).
• make "single view" step clearer; this is missing from the extraction step, and should be explained separately anyway.

The keys in the data file should also be explained somewhere, This could be done with a mapping from the terms in section 3.2 (e.g. H(v, l, e, s)) to the keys (e.g. brirs).