2.x composite disposable docs

[chronvas]

On CompositeDisposable add and addAll methods, if the param is null, currently the NPE error message (produced by ObjectHelper) is "d is null" which is not very helpful.

This is a small refactor for making the message a bit more helpful.

Resolves #6430

[akarnokd]

If you want to improve, change the argument name to disposable instead.

[akarnokd]

Please rename the arguments/variables instead.

[codecov]

Codecov Report

Merging #6431 into 2.x will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #6431      +/-   ##
===========================================
- Coverage     98.34%   98.3%   -0.05%     
+ Complexity     6300    6297       -3     
===========================================
  Files           675     675              
  Lines         45172   45172              
  Branches       6246    6246              
===========================================
- Hits          44425   44406      -19     
- Misses          229     235       +6     
- Partials        518     531      +13

Impacted Files	Coverage Δ	Complexity Δ
.../io/reactivex/disposables/CompositeDisposable.java	100% <100%> (ø)	40 <4> (ø)	⬇️
...ternal/operators/observable/ObservablePublish.java	93.8% <0%> (-4.43%)	10% <0%> (-1%)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.36% <0%> (-3.2%)	9% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	96.2% <0%> (-2.11%)	11% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...internal/operators/observable/ObservableCache.java	95.9% <0%> (-1.64%)	34% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
... and 16 more

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[chronvas]

Sorry, went ahead and created another branch before you replied. Will close this one, please have a look at PR #6432