1.x: add missing backpressure descriptions and update old ones

[akarnokd]

This PR adds or updates places where the Backpressure: description was missing or outdated.

Unfortunately, GitHub has a hard limit on diff size per file so reviewers have to use their own local diff.

[DavidMGross]

"...it <t>may</i> lead to IllegalStateException being throw..."
=> "...it <em>may</em> lead to IllegalStateException being thrown..."

...but better yet would be to avoid the passive voice and indicate what entity throws the exception, e.g. "If any of the source Observables violate this, XXX may throw an IllegalStateException when the source Observable completes."

[DavidMGross]

"...no backpressue applied to them..." => "...no backpressure applied to them..."

[DavidMGross]

"The other Observables supplied by the function is consumed..." => "The other Observables supplied by the function are consumed..."

[DavidMGross]

I notice that some of the <dd>s aren't being closed with </dd> also.

[DavidMGross]

typo: "...apply any of the onBackpressureXXX opertors <b>before</b> applying observeOn itself..." => "...apply any of the onBackpressureXXX operators <strong>before</strong> applying observeOn itself..."

[DavidMGross]

For operators that both accept and return an Observable (most of them), I find the phrase "this Observable" in the documentation to be ambiguous: does it mean the Observable about to be operated on, or the Observable that results from the operation? I'd recommend replacing it with "the source Observable" or "the resulting Observable" or something like that, depending on which one you're talking about.

[DavidMGross]

typo: "...the output's backpressure behavior is derermined by..." => "...the output's backpressure behavior is determined by..."

[DavidMGross]

+     *  <dt><b>Backpressure:</b></dt>
+     *  <dd>The operator honors backpressure from downstream and consumes this {@code Observable} in an
+     *  unbounded manner (i.e., no backpressure is applied to it).</dd>
+     *  behavior.</dd>

That last line seems to be extraneous.

[DavidMGross]

Some of those typos appear in multiple places in the file.

It's great to see these sections getting filled out and made more precise!

[akarnokd]

Thanks for the review. I've updated the text.

[codecov-io]

Current coverage is 81.46%

Merging #4172 into 1.x will increase coverage by 0.04%

@@                1.x      #4172   diff @@
==========================================
  Files           257        257          
  Lines         16823      16823          
  Methods           0          0          
  Messages          0          0          
  Branches       2550       2550          
==========================================
+ Hits          13697      13705     +8   
+ Misses         2226       2225     -1   
+ Partials        900        893     -7   

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[akarnokd]

I'm merging this so work touching Observable.java can commence. Anybody let me know if you find something else (or better yet, post a PR).

[artem-zinnatullin]

Thank you for this @akarnokd!

[davidmoten]

Beaut!