2.x: enable link to external JDK, fix Schedulers style

[akarnokd]

This PR enables the generated Javadoc to {@link } to JDK classes properly (current 2.0.8 doc shows them as plain text). I'm linking to the v7 docs because v6 is ugly.

In addition, I've fixed the style of the Schedulers documentation by separating the Supported system properties properly into a new paragraph.

[codecov]

Codecov Report

Merging #5271 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #5271      +/-   ##
===========================================
+ Coverage       96%   96.04%   +0.03%     
- Complexity    5749     5754       +5     
===========================================
  Files          628      628              
  Lines        41077    41077              
  Branches      5699     5699              
===========================================
+ Hits         39437    39453      +16     
+ Misses         660      645      -15     
+ Partials       980      979       -1

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	10 <0> (ø)	⬇️
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...reactivex/internal/operators/maybe/MaybeUsing.java	93.93% <0%> (-4.05%)	4% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.54% <0%> (-3.99%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...main/java/io/reactivex/subjects/SingleSubject.java	95.23% <0%> (-2.39%)	38% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	93.04% <0%> (-2.14%)	5% <0%> (ø)
...a/io/reactivex/processors/SerializedProcessor.java	95.74% <0%> (-2.13%)	27% <0%> (-1%)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
...activex/internal/disposables/DisposableHelper.java	98.14% <0%> (-1.86%)	27% <0%> (-1%)
... and 32 more

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[JakeWharton]

the docs for 8 look even nicer if you want!

[akarnokd]

The Travis build runs on Java 7 so the javadoc style is v7 - giving a consistent view when jumping the links.