Improve documentation of new features

[tsalo]

Closes none.

Changes proposed in this pull request

• Explicitly limit atlas query to discrete segmentations (suffix = dseg). Probabilistic segmentations could be possible in the future, but the current parcellation code wouldn't work on them.
• Format code blocks in the documentation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.80%. Comparing base (90dc40a) to head (704e810).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1281   +/-   ##
=======================================
  Coverage   78.80%   78.80%           
=======================================
  Files          59       59           
  Lines        7359     7360    +1     
  Branches     1136     1136           
=======================================
+ Hits         5799     5800    +1     
  Misses       1267     1267           
  Partials      293      293           

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