remove cell_size docs from RNN

[cgarciae]

What does this PR do?

• Remove the cell_size entry from the RNN docstring.

[codecov-commenter]

Codecov Report

Merging #3186 (aac2f05) into main (1e7a8b1) will decrease coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #3186      +/-   ##
==========================================
- Coverage   82.30%   82.30%   -0.01%     
==========================================
  Files          54       54              
  Lines        6054     6059       +5     
==========================================
+ Hits         4983     4987       +4     
- Misses       1071     1072       +1     

Impacted Files	Coverage Δ
flax/linen/normalization.py	96.71% <80.00%> (-0.61%)	⬇️
flax/linen/recurrent.py	97.28% <100.00%> (+0.01%)	⬆️