Document some transactional requirements on some primitives

[realityforge]

No description provided.

[codecov-io]

Codecov Report

Merging #42 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #42   +/-   ##
=========================================
  Coverage     90.75%   90.75%           
  Complexity     3103     3103           
=========================================
  Files            92       92           
  Lines          7379     7379           
  Branches       1230     1230           
=========================================
  Hits           6697     6697           
  Misses          132      132           
  Partials        550      550

Impacted Files	Coverage Δ	Complexity Δ
core/src/main/java/arez/ObservableValue.java	87.73% <ø> (ø)	140 <0> (ø)	⬇️
core/src/main/java/arez/ComputedValue.java	86.88% <ø> (ø)	66 <0> (ø)	⬇️

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