Improve doOnDispose JavaDoc

[Mauin]

Sorry for the earlier PR #5295... :/

This addresses the JavaDoc issues discussed in #5283.

• Removes the Note in Observable.doOnDispose which claims its action would be called on terminal events.
• Slightly adjusts the JavaDocs of the Maybe/Single/Completable variants.
• Adds @throws note to some Observable/Single/Maybe .doOnDispose as it was present in the Completable variant.

[codecov]

Codecov Report

Merging #5296 into 2.x will increase coverage by 0.15%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5296      +/-   ##
============================================
+ Coverage     96.11%   96.26%   +0.15%     
- Complexity     5750     5767      +17     
============================================
  Files           628      628              
  Lines         41085    41085              
  Branches       5703     5703              
============================================
+ Hits          39487    39551      +64     
+ Misses          634      600      -34     
+ Partials        964      934      -30

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	112 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	99.32% <ø> (ø)	131 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	168 <0> (ø)	⬇️
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <0%> (-2.69%)	7% <0%> (ø)
...al/operators/observable/ObservableSampleTimed.java	96.66% <0%> (-1.67%)	3% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	95.52% <0%> (-0.75%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	82.89% <0%> (-0.66%)	3% <0%> (ø)
... and 33 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 5ab2083...6c6c8f4. Read the comment docs.