1.x: add marble diagram to UnicastSubject, fix javadoc

[akarnokd]

This PR fixes a missing javadoc entry for <T> in UnicastSubject and adds a marble diagram to the class itself:

[codecov]

Codecov Report

Merging #5209 into 1.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                1.x    #5209      +/-   ##
============================================
+ Coverage     84.38%   84.39%   +0.01%     
- Complexity     2882     2883       +1     
============================================
  Files           290      290              
  Lines         18123    18123              
  Branches       2479     2479              
============================================
+ Hits          15293    15295       +2     
+ Misses         1963     1956       -7     
- Partials        867      872       +5

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/rx/subjects/UnicastSubject.java	84.56% <ø> (ø)	9 <0> (ø)	⬇️
...ain/java/rx/internal/operators/OnSubscribeAmb.java	79.13% <0%> (-5.04%)	13% <0%> (ø)
...ain/java/rx/internal/schedulers/SchedulerWhen.java	83.78% <0%> (-4.06%)	4% <0%> (ø)
...rx/internal/operators/OnSubscribeFromIterable.java	91.46% <0%> (-2.44%)	5% <0%> (ø)
.../rx/internal/schedulers/CachedThreadScheduler.java	87.37% <0%> (-1.95%)	6% <0%> (ø)
...n/java/rx/subscriptions/CompositeSubscription.java	75.32% <0%> (-1.3%)	24% <0%> (ø)
...in/java/rx/internal/operators/OperatorGroupBy.java	90.81% <0%> (-0.71%)	5% <0%> (ø)
.../java/rx/internal/operators/BackpressureUtils.java	70.45% <0%> (+2.27%)	28% <0%> (+1%)	⬆️
.../rx/internal/operators/OperatorWindowWithTime.java	48.48% <0%> (+6.43%)	3% <0%> (ø)	⬇️

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