1.x: Improve the BehaviorSubject Java doc

[BraisGabin]

• The second example didn't explain what happened with "default".
• The first line is copied from the 2.x documentation. It's more clear.

[codecov]

Codecov Report

Merging #5048 into 1.x will increase coverage by 0.01%.

@@             Coverage Diff              @@
##                1.x    #5048      +/-   ##
============================================
+ Coverage     84.17%   84.19%   +0.01%     
- Complexity        0     2858    +2858     
============================================
  Files           288      288              
  Lines         17805    17805              
  Branches       2698     2420     -278     
============================================
+ Hits          14988    14991       +3     
+ Misses         1958     1957       -1     
+ Partials        859      857       -2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/rx/subjects/BehaviorSubject.java	97.1% <ø> (ø)	34 <ø> (+34)	✅
...main/java/rx/internal/operators/OperatorMerge.java	86.37% <ø> (-0.47%)	7% <ø> (+7%)
...in/java/rx/internal/operators/OperatorPublish.java	78.48% <ø> (-0.43%)	8% <ø> (+8%)
...n/java/rx/subjects/SubjectSubscriptionManager.java	82.14% <ø> (+1.42%)	16% <ø> (+16%)	✅
.../java/rx/internal/operators/BackpressureUtils.java	72.72% <ø> (+2.27%)	29% <ø> (+29%)	✅
...n/java/rx/subscriptions/CompositeSubscription.java	76.62% <ø> (+2.59%)	24% <ø> (+24%)	✅

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