Add missing doc comments for time slices and processes

[alexdewar]

Description

This PR adds missing doc comments for time slice- and process-related code. For the time slice code, there's also a small amount of rearranging, but no functional changes are intended anywhere.

Closes #220. Closes #222.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.58%. Comparing base (637d49a) to head (eb7af1f).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
+ Coverage   95.56%   95.58%   +0.02%     
==========================================
  Files          31       31              
  Lines        4122     4121       -1     
  Branches     4122     4121       -1     
==========================================
  Hits         3939     3939              
+ Misses         91       90       -1     
  Partials       92       92              

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[TinyMarsh]

LGTM