2.x: fix Javadoc mistakes and some style

[akarnokd]

This PR fixes most JavaDoc errors discovered from RxJava 3 preview's javadoc generator (doclint enabled).

(I was unable to make gradle in RxJava 2 to report the same errors regardless of source level and underlying Java 8 runtime locally.)

Changes include:

• Fix closing tag of <dt> in <dl>s
• Fix <td> to <dt> in <dl>s
• Replace > in <code> blocks with > (as it is forbidden in doclint of Java 8)
• Replace < in <code> blocks with <
• Add source code checker for the previous mistakes
• Remove initialSeed mentions from scan operators that take a seedSupplier and remove the deferred example as well
• Switch the order of <code><pre> to <pre><code>.
• A couple of end-sentence punctuation.

[codecov]

Codecov Report

Merging #5460 into 2.x will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5460      +/-   ##
===========================================
- Coverage     96.09%     96%   -0.09%     
  Complexity     5797    5797              
===========================================
  Files           631     631              
  Lines         41306   41306              
  Branches       5745    5745              
===========================================
- Hits          39694   39657      -37     
- Misses          624     643      +19     
- Partials        988    1006      +18

Impacted Files	Coverage Δ	Complexity Δ
...a/io/reactivex/subscribers/ResourceSubscriber.java	100% <ø> (ø)	10 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	523 <0> (ø)	⬇️
...va/io/reactivex/subscribers/DefaultSubscriber.java	100% <ø> (ø)	7 <0> (ø)	⬇️
...ava/io/reactivex/observers/DisposableObserver.java	100% <ø> (ø)	7 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	132 <0> (ø)	⬇️
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	10 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
...io/reactivex/subscribers/DisposableSubscriber.java	100% <ø> (ø)	9 <0> (ø)	⬇️
.../io/reactivex/observers/ResourceMaybeObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	112 <0> (ø)	⬇️
... and 55 more

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