Adding documentation to Dropout around rng use

[zaxtax]

Hey, I've been playing with the flax project lately and this is something that tripped me up. I believe I followed the correct documentation standards and hope it makes Dropout 's behaviour more clear.

Fixes #1004

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.

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[codecov-commenter]

Codecov Report

Merging #2492 (ee21f85) into main (cc88a73) will decrease coverage by 0.04%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##             main    #2492      +/-   ##
==========================================
- Coverage   78.81%   78.77%   -0.05%     
==========================================
  Files          48       48              
  Lines        5066     5069       +3     
==========================================
  Hits         3993     3993              
- Misses       1073     1076       +3     

Impacted Files	Coverage Δ
flax/linen/module.py	92.79% <ø> (ø)
flax/linen/stochastic.py	96.55% <ø> (ø)
flax/training/checkpoints.py	60.93% <40.00%> (-0.43%)	⬇️
flax/serialization.py	69.09% <100.00%> (-0.19%)	⬇️
flax/training/dynamic_scale.py	0.00% <0.00%> (ø)

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[cgarciae]

Checking the preview, seems that the reference is not being picked up:
https://flax--2492.org.readthedocs.build/en/2492/api_reference/_autosummary/flax.linen.Dropout.html?highlight=dropout#flax.linen.Dropout