Update pori docs to include comparator ('analysis role') name definitions

[elewis2]

Update the docs to accompany the added comparator type 'internal pancancer': https://bcgsc.github.io/pori/ipr/core_variants/expression/#comparators

Perhaps we can add an exhaustive list of definitions for each of the comparators here: https://bcgsc.github.io/pori/ipr/optional_analyses/comparators/

(paraphrased from linked comment by @creisle)

Originally posted by @creisle in bcgsc/pori_ipr_python#82 (comment)