2.x: fix wording of toList, fix a/an in subscribeOn

[akarnokd]

This PR

• fixes the wording of toList() putting emphasis of the need for finite sources and removes a misleading/incorrect sentence about unstoppability;
• corrects a/an spelling errors in subscribeOn.
• removes the mention of toBlocking from the toFuture javadoc

[codecov]

Codecov Report

Merging #5465 into 2.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5465      +/-   ##
============================================
- Coverage     95.94%   95.89%   -0.06%     
+ Complexity     5797     5794       -3     
============================================
  Files           631      631              
  Lines         41306    41306              
  Branches       5745     5745              
============================================
- Hits          39633    39610      -23     
- Misses          656      668      +12     
- Partials       1017     1028      +11

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	523 <0> (ø)	⬇️
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-4.92%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	87.43% <0%> (-4.84%)	2% <0%> (ø)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	94.11% <0%> (-4.42%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	94.87% <0%> (-3.42%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
... and 37 more

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