Document creating Multis from a generator

[jponge]

See #866

[codecov]

Codecov Report

Merging #867 (18bec32) into main (c4fd4bf) will increase coverage by 0.19%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #867      +/-   ##
============================================
+ Coverage     89.10%   89.29%   +0.19%     
- Complexity     3071     3078       +7     
============================================
  Files           386      386              
  Lines         12354    12354              
  Branches       1574     1576       +2     
============================================
+ Hits          11008    11032      +24     
+ Misses          705      692      -13     
+ Partials        641      630      -11     

Impacted Files	Coverage Δ
...in/java/io/smallrye/mutiny/groups/MultiReplay.java	100.00% <ø> (ø)
.../io/smallrye/mutiny/helpers/queues/DrainUtils.java	77.77% <0.00%> (-2.23%)	⬇️
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	84.80% <0.00%> (-0.92%)	⬇️
...lrye/mutiny/subscription/SerializedSubscriber.java	86.23% <0.00%> (+0.91%)	⬆️
...a/io/smallrye/mutiny/helpers/BlockingIterable.java	87.03% <0.00%> (+0.92%)	⬆️
...tiny/operators/multi/MultiBufferWithTimeoutOp.java	74.78% <0.00%> (+1.68%)	⬆️
...io/smallrye/mutiny/operators/uni/UniMemoizeOp.java	93.20% <0.00%> (+1.94%)	⬆️
...allrye/mutiny/operators/uni/UniAndCombination.java	90.19% <0.00%> (+1.96%)	⬆️
...mallrye/mutiny/operators/multi/FlatMapManager.java	98.61% <0.00%> (+2.77%)	⬆️
... and 4 more