Update RPC input output documentation (#435)

[shgandhi]

• Updates documentation to correctly describe RPC input/output

[codecov-io]

Codecov Report

Merging #458 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   83.57%   83.57%   +<.01%     
==========================================
  Files         109      109              
  Lines       13103    13104       +1     
  Branches     1923     1923              
==========================================
+ Hits        10951    10952       +1     
  Misses       1593     1593              
  Partials      559      559

Impacted Files	Coverage Δ
ydkgen/printer/doc/doc_printer.py	90.9% <100%> (+0.76%)	⬆️
ydkgen/builder/test_case/test_cases_builder.py	80.93% <0%> (-1.08%)	⬇️
ydkgen/builder/test_case/test_value_builder.py	92.05% <0%> (+0.83%)	⬆️

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