xMr/do on dispose javadoc

[Mauin]

Thank you for contributing to RxJava. Before pressing the "Create Pull Request" button, please consider the following points:

• Please give a description about what and why you are contributing, even if it's trivial.

• Please include the issue list number(s) or other PR numbers in the description if you are contributing in response to those.

• Please include a reasonable set of unit tests if you contribute new code or change an existing one. If you contribute an operator, (if applicable) please make sure you have tests for working with an empty, just, range of values as well as an error source, with and/or without backpressure and see if unsubscription/cancellation propagates correctly.

[codecov]

Codecov Report

Merging #5295 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5295      +/-   ##
============================================
+ Coverage     96.11%   96.11%   +<.01%     
- Complexity     5750     5758       +8     
============================================
  Files           628      628              
  Lines         41085    41085              
  Branches       5703     5703              
============================================
+ Hits          39487    39490       +3     
+ Misses          634      631       -3     
  Partials        964      964

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)	⬇️
...ternal/operators/flowable/FlowableSampleTimed.java	89.7% <0%> (-5.89%)	3% <0%> (ø)
...vex/internal/subscriptions/SubscriptionHelper.java	94.36% <0%> (-5.64%)	34% <0%> (-2%)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.97% <0%> (-4.69%)	57% <0%> (+1%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <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)
... and 34 more

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