2.x: Improve subjects and processors package doc

[akarnokd]

• Fix a typo in the subjects doc
• Expand the processors doc

[codecov]

Codecov Report

Merging #6434 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6434      +/-   ##
============================================
+ Coverage     98.24%   98.26%   +0.01%     
- Complexity     6294     6296       +2     
============================================
  Files           675      675              
  Lines         45172    45172              
  Branches       6246     6246              
============================================
+ Hits          44380    44389       +9     
+ Misses          250      242       -8     
+ Partials        542      541       -1

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-6.54%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowableFlatMap.java	88.42% <0%> (-1.32%)	4% <0%> (ø)
...activex/internal/schedulers/ExecutorScheduler.java	95.97% <0%> (-1.01%)	10% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	96.2% <0%> (-0.43%)	11% <0%> (ø)
... and 19 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0bb7b4d...9f8f1ee. Read the comment docs.