Add some notes/caveats in WorkStealingDeque docs

[tkf]

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[codecov-commenter]

Codecov Report

Merging #49 (e5ec72b) into master (dca80c7) will decrease coverage by 2.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   89.04%   86.17%   -2.88%     
==========================================
  Files          14       14              
  Lines        1433     1432       -1     
==========================================
- Hits         1276     1234      -42     
- Misses        157      198      +41     

Flag	Coverage Δ
unittests	86.17% <ø> (-2.88%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/promise.jl	70.37% <0.00%> (-18.52%)	⬇️
src/stack.jl	89.28% <0.00%> (-7.15%)	⬇️
src/utils.jl	64.51% <0.00%> (-5.65%)	⬇️
src/lcrq.jl	88.95% <0.00%> (-4.07%)	⬇️
src/dlcrq.jl	85.79% <0.00%> (-3.48%)	⬇️
src/msqueue.jl	85.41% <0.00%> (-2.09%)	⬇️
src/dict.jl	88.02% <0.00%> (-1.59%)	⬇️
src/ssqueue.jl	95.74% <0.00%> (-0.71%)	⬇️

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