Improve documentation of Flux.merge

[glyn]

Avoid talking about "inner sources" or "inner publishers" when there is only
one collection of sources in sight. Reserve the use of the adjective "inner"
for variants of Flux.merge which deal with publishers of publishers.

Also, use the term "sources" instead of "publishers" consistently across the
related variants of Flux.merge.

[codecov-io]

Codecov Report

Merging #1020 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1020      +/-   ##
============================================
- Coverage     83.53%   83.45%   -0.09%     
+ Complexity     3428     3425       -3     
============================================
  Files           331      331              
  Lines         26932    26932              
  Branches       4985     4985              
============================================
- Hits          22497    22475      -22     
- Misses         2949     2964      +15     
- Partials       1486     1493       +7

Impacted Files	Coverage Δ	Complexity Δ
...ore/src/main/java/reactor/core/publisher/Flux.java	100% <ø> (ø)	481 <0> (ø)	⬇️
.../java/reactor/util/concurrent/MpscLinkedQueue.java	80.26% <0%> (-7.9%)	15% <0%> (-2%)
.../java/reactor/core/publisher/BlockingIterable.java	70.52% <0%> (-6.32%)	6% <0%> (ø)
...ain/java/reactor/util/concurrent/WaitStrategy.java	51.21% <0%> (-1.63%)	11% <0%> (ø)
...in/java/reactor/core/publisher/FluxWindowWhen.java	80.67% <0%> (-1.45%)	2% <0%> (ø)
...eactor/core/publisher/ParallelMergeSequential.java	79.79% <0%> (-1.04%)	7% <0%> (ø)
...ain/java/reactor/core/publisher/FluxPublishOn.java	87.63% <0%> (-0.63%)	5% <0%> (ø)
...rc/main/java/reactor/core/publisher/Operators.java	71.66% <0%> (-0.55%)	80% <0%> (-1%)
...ava/reactor/core/publisher/EventLoopProcessor.java	80.2% <0%> (-0.53%)	52% <0%> (-1%)
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.18% <0%> (-0.41%)	13% <0%> (ø)
... and 5 more

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