[MINOR]: Add documentation to datafusion-row modules

[alamb]

Rationale:

I am working on writing up more detail on various next steps for the RowFormat on #1861 and found myself reading the code to understand what was going on better. I have encoded various thoughts into docstrings so hopefully future readers will find the code easier to navigate.

Changes:

• Add / move docstrings
• No code change

cc @yjshen

[yjshen]

Great writing and nice diagrams. Thanks @alamb!

[codecov-commenter]

Codecov Report

Merging #2704 (6543d75) into master (bbb674a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2704   +/-   ##
=======================================
  Coverage   84.66%   84.66%           
=======================================
  Files         270      270           
  Lines       46919    46919           
=======================================
+ Hits        39723    39726    +3     
+ Misses       7196     7193    -3     

Impacted Files	Coverage Δ
datafusion/row/src/accessor.rs	56.41% <ø> (ø)
datafusion/row/src/layout.rs	98.27% <ø> (ø)
datafusion/row/src/lib.rs	100.00% <ø> (ø)
datafusion/row/src/reader.rs	88.27% <ø> (ø)
datafusion/row/src/writer.rs	89.86% <ø> (ø)
datafusion/optimizer/src/utils.rs	32.25% <0.00%> (-0.54%)	⬇️
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/common/src/scalar.rs	74.94% <0.00%> (+0.11%)	⬆️
datafusion/expr/src/logical_plan/plan.rs	74.75% <0.00%> (+0.59%)	⬆️
datafusion/expr/src/window_frame.rs	93.27% <0.00%> (+0.84%)	⬆️

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