Document slice DSTs and size validity

[joshlf]

Closes #1263

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.22%. Comparing base (61502d3) to head (6ac2e10).
Report is 512 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1273   +/-   ##
=======================================
  Coverage   88.22%   88.22%           
=======================================
  Files          14       14           
  Lines        5131     5131           
=======================================
  Hits         4527     4527           
  Misses        604      604           

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