2.x: Document and test amb subscription ordering.

[JakeWharton]

Refs #5044.

[JakeWharton]

Once wording and tests are approved I'll port to 1.x as well.

[akarnokd]

Nice!

(Forgot about testing all signal types as well, so 5 x 3 x 3, but I don't require it.)

[JakeWharton]

Hmm I seem to be fighting the cool documentation checker which is being a bit too clever in this instance. I'm using the word "Subscription" at the beginning of a sentence which it thinks means Subscription the type. I'll figure out a wording tweak to make it end up as a lowercase "subscription".

[JakeWharton]

Also with regard to signal types, I figure the other tests cover those already so in these cases we're just guarding against ordering changes.

[akarnokd]

Yes, lowercase it as Subscription is interpreted as the reference to the type.

[codecov]

Codecov Report

Merging #5047 into 2.x will increase coverage by 0.08%.

@@             Coverage Diff             @@
##                2.x   #5047      +/-   ##
===========================================
+ Coverage     95.52%   95.6%   +0.08%     
- Complexity     5539    5541       +2     
===========================================
  Files           612     612              
  Lines         39576   39576              
  Branches       5553    5553              
===========================================
+ Hits          37804   37838      +34     
+ Misses          773     758      -15     
+ Partials        999     980      -19

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	112 <ø> (ø)	❌
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	167 <ø> (ø)	❌
src/main/java/io/reactivex/Single.java	99.29% <ø> (ø)	130 <ø> (ø)	❌
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	504 <ø> (ø)	❌
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	520 <ø> (ø)	❌
...ternal/operators/flowable/FlowableSubscribeOn.java	90.9% <ø> (-7.28%)	2% <ø> (ø)
...nternal/operators/observable/ObservableCreate.java	92.85% <ø> (-5.36%)	2% <ø> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	93.87% <ø> (-4.09%)	2% <ø> (ø)
...ternal/operators/observable/ObservablePublish.java	91.89% <ø> (-3.61%)	10% <ø> (ø)
.../operators/observable/ObservableFlatMapSingle.java	92.53% <ø> (-2.99%)	2% <ø> (ø)
... and 37 more

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