3.x: Improve wording of operators

[akarnokd]

• Reword sentences with "modifies" and "instructs".
• Reword "the source X" to "the current X".
• Reword a couple of other operator sentences.

Related #6819
Resolves #6821

[codecov]

Codecov Report

Merging #6843 into 3.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6843      +/-   ##
============================================
+ Coverage     98.25%   98.28%   +0.02%     
- Complexity     6380     6381       +1     
============================================
  Files           715      715              
  Lines         46529    46529              
  Branches       6388     6388              
============================================
+ Hits          45717    45730      +13     
+ Misses          293      286       -7     
+ Partials        519      513       -6

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	576 <0> (ø)	⬇️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	179 <0> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	151 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	551 <0> (ø)	⬇️
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	89.68% <0%> (-1.59%)	2% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	95.59% <0%> (-1.26%)	3% <0%> (ø)
...ivex/rxjava3/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ava3/internal/operators/maybe/MaybeMergeArray.java	96.04% <0%> (-1.13%)	6% <0%> (ø)
... and 16 more

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