Improve Sortition Documentation

[fed-franz]

This PR adds/improves documentation of Deterministic Sortition code.

---

Resolves #1491

[codecov-commenter]

Codecov Report

Base: 40.18% // Head: 40.14% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (79b1346) compared to base (57c6bbd).
Patch coverage: 19.46% of modified lines in pull request are covered.

❗ Current head 79b1346 differs from pull request most recent head 3b7351c. Consider uploading reports for the commit 3b7351c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1493      +/-   ##
==========================================
- Coverage   40.18%   40.14%   -0.04%     
==========================================
  Files         147      147              
  Lines        9651     9737      +86     
==========================================
+ Hits         3878     3909      +31     
- Misses       5144     5204      +60     
+ Partials      629      624       -5     

Impacted Files	Coverage Δ
pkg/config/genesis/config.go	0.00% <ø> (ø)
pkg/config/genesis/presets.go	69.23% <ø> (ø)
...nsensus/blockgenerator/candidate/blockgenerator.go	0.00% <0.00%> (ø)
...core/consensus/blockgenerator/candidate/genesis.go	0.00% <0.00%> (ø)
...kg/core/consensus/blockgenerator/candidate/mock.go	0.00% <0.00%> (ø)
pkg/core/consensus/user/sortition.go	58.25% <ø> (ø)
pkg/core/data/ipc/transactions/fixtures.go	13.76% <0.00%> (-0.21%)	⬇️
pkg/core/data/ipc/transactions/provider.go	0.00% <0.00%> (ø)
pkg/gql/query/blocks.go	63.22% <0.00%> (ø)
pkg/gql/query/mempool.go	30.00% <0.00%> (ø)
... and 12 more

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[herr-seppia]

LGTM

just 2 suggestion to be consistent between comments format