Document mutability and metaclass interactions

[antonagestam]

Closes #127.
Closes #128.

[codecov-commenter]

Codecov Report

Merging #154 (c43cc22) into main (f84b3be) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #154   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines          669       669           
  Branches        51        51           
=========================================
  Hits           669       669           

Impacted Files	Coverage Δ
src/phantom/__init__.py	100.00% <ø> (ø)
src/phantom/interval.py	100.00% <ø> (ø)
src/phantom/iso3166.py	100.00% <ø> (ø)
src/phantom/re.py	100.00% <ø> (ø)
src/phantom/schema.py	100.00% <ø> (ø)
src/phantom/sized.py	100.00% <ø> (ø)

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