Clarify backpressure overflow error message of interval

[simonbasle]

This commit clarifies the javadoc and the error message propagated when
a Flux#interval ticks beyond downstream requests, resulting in an
OverflowException.

The message and javadoc make it more explicit that the error is coming
from the fact that interval doesn't really support backpressure, and
that the downstream should replenish requested amount before all the
ticks have been emitted.

[codecov-io]

Codecov Report

Merging #911 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #911   +/-   ##
=========================================
  Coverage     83.67%   83.67%           
- Complexity     3312     3314    +2     
=========================================
  Files           323      323           
  Lines         26173    26173           
  Branches       4857     4857           
=========================================
  Hits          21901    21901           
- Misses         2817     2818    +1     
+ Partials       1455     1454    -1

Impacted Files	Coverage Δ	Complexity Δ
...ore/src/main/java/reactor/core/publisher/Flux.java	100% <ø> (ø)	473 <0> (ø)	⬇️
...main/java/reactor/core/publisher/FluxInterval.java	80.95% <100%> (+7.14%)	2 <0> (ø)	⬇️
.../java/reactor/core/publisher/FluxLimitRequest.java	94% <0%> (-4%)	5% <0%> (ø)
...ava/reactor/core/publisher/WorkQueueProcessor.java	69.06% <0%> (-1.28%)	19% <0%> (ø)
...ain/java/reactor/core/publisher/FluxPublishOn.java	87.84% <0%> (-0.42%)	5% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxCreate.java	86.25% <0%> (-0.26%)	8% <0%> (ø)
...ava/reactor/core/publisher/EventLoopProcessor.java	78.68% <0%> (+0.54%)	50% <0%> (+1%)	⬆️
...main/java/reactor/core/publisher/FluxGenerate.java	70.37% <0%> (+0.61%)	7% <0%> (ø)	⬇️
...a/reactor/core/publisher/FluxBufferTimeOrSize.java	74.6% <0%> (+0.79%)	2% <0%> (ø)	⬇️
...rc/main/java/reactor/core/publisher/FluxRange.java	84.66% <0%> (+1.33%)	5% <0%> (ø)	⬇️
... and 1 more

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[simonbasle]

PR 911, CALL THE PO-LICE 👮 👮‍♀️ 🚔