[doc] Improve doOnDiscard/onErrorContinue javadoc a bit

[simonbasle]

No description provided.

[codecov-io]

Codecov Report

Merging #2463 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2463      +/-   ##
============================================
- Coverage     83.69%   83.67%   -0.03%     
  Complexity     4699     4699              
============================================
  Files           389      389              
  Lines         32214    32214              
  Branches       6196     6196              
============================================
- Hits          26962    26954       -8     
- Misses         3543     3545       +2     
- Partials       1709     1715       +6     

Impacted Files	Coverage Δ	Complexity Δ
...ore/src/main/java/reactor/core/publisher/Flux.java	98.09% <ø> (ø)	533.00 <0.00> (ø)
...ore/src/main/java/reactor/core/publisher/Mono.java	90.73% <ø> (ø)	283.00 <0.00> (ø)
...actor/core/publisher/FluxOnBackpressureLatest.java	81.00% <0.00%> (-3.00%)	4.00% <0.00%> (ø%)
...ava/reactor/core/publisher/FluxWindowBoundary.java	80.44% <0.00%> (-1.68%)	4.00% <0.00%> (ø%)
...in/java/reactor/core/publisher/FluxWindowWhen.java	84.43% <0.00%> (-0.95%)	4.00% <0.00%> (ø%)
.../java/reactor/core/publisher/UnicastProcessor.java	90.95% <0.00%> (-0.91%)	87.00% <0.00%> (-1.00%)
...java/reactor/core/scheduler/ExecutorScheduler.java	79.16% <0.00%> (-0.70%)	12.00% <0.00%> (ø%)
...ain/java/reactor/core/publisher/FluxConcatMap.java	90.41% <0.00%> (-0.55%)	9.00% <0.00%> (ø%)
...ain/java/reactor/core/publisher/FluxPublishOn.java	89.69% <0.00%> (-0.19%)	8.00% <0.00%> (ø%)
...c/main/java/reactor/core/publisher/FluxReplay.java	84.87% <0.00%> (+0.14%)	30.00% <0.00%> (ø%)
... and 4 more

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