Add comment on multiprocessing.

[vincentmr]

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---

Context:
Multiprocessing sometimes fails without much clue.

Description of the Change:
Add a warning in the DefaultQubit doc regarding potential erratic behaviour of multiprocessing and a possible to solve it.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f56e4fe) 99.63% compared to head (adc14a7) 99.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4622   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files         375      375           
  Lines       33547    33547           
=======================================
  Hits        33423    33423           
  Misses        124      124           

Files Changed	Coverage Δ
pennylane/devices/default_qubit.py	100.00% <ø> (ø)

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[mudit2812]

Looks good, just one comment. I'll approve after it's addressed.

[timmysilv]

content looks excellent, but I think DefaultQubit's intro section is getting a bit too lengthy. Can you move the multiprocessing stuff below the backprop stuff, and put it into a drop-down (ps: drop-downs can be made like this in sphinx)?

[timmysilv]

🎉 love it