2.x: Fix Javadoc warnings, links to the JDK types

[akarnokd]

The definition of multiple external links was wrong in build.gradle where the options.links() is actually defined as String... and multiple calls are not additional. This made the generated JavaDoc not have links to the JDK types. The fix now properly uses the varargs of the method. In addition, somehow the plain http:// still wouldn't generate the proper links probably because they are redirected to https:// for which the javadoc tool is not prepared.

In addition, 4 dangling <p> tags were showing up as warnings and have been removed.

[JakeWharton]

Any reason not to use 8 or 9?

[akarnokd]

No particular reason.

[artem-zinnatullin]

Since RxJava 2.x is compatible with JDK 6, should we link 6?

[akarnokd]

Well, the Javadoc is built with Java 8 and the JDK 6 docs look ugly to me anyway. Style-wise, 8 would be more seamless, 9 would be nicer due to the search box. Anything 7+ may trigger confusion about the JDK's default methods not available in Android or the misunderstanding that it's only the docs that link to Java 9, not that RxJava now depends on Java 9.

[codecov]

Codecov Report

Merging #5861 into 2.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5861      +/-   ##
============================================
- Coverage     96.54%   96.49%   -0.05%     
+ Complexity     5858     5850       -8     
============================================
  Files           646      646              
  Lines         42640    42640              
  Branches       5910     5910              
============================================
- Hits          41166    41145      -21     
- Misses          566      577      +11     
- Partials        908      918      +10

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.24% <0%> (-5.83%)	54% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-5.09%)	10% <0%> (-1%)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.78% <0%> (-3.39%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
... and 28 more

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[artem-zinnatullin]

Since RxJava 2.x is compatible with JDK 6, should we link 6?