Update area units and available_area description.

[brynpickering]

Fixes #701

Summary of changes in this pull request

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.97%. Comparing base (ed25596) to head (f1cc45e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #705    +/-   ##
========================================
  Coverage   95.97%   95.97%            
========================================
  Files          29       29            
  Lines        4048     4048            
  Branches      849      578   -271     
========================================
  Hits         3885     3885            
  Misses         72       72            
  Partials       91       91            

see 2 files with indirect coverage changes