Added more documentation

[jorgecarleitao]

Also added #![deny(missing_docs)] to some of the modules so that we start forcing ourselves to produce API docs for public APIs.

[codecov]

Codecov Report

Merging #476 (e759a86) into main (76e8992) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #476   +/-   ##
=======================================
  Coverage   80.08%   80.08%           
=======================================
  Files         373      373           
  Lines       22937    22937           
=======================================
  Hits        18370    18370           
  Misses       4567     4567           

Impacted Files	Coverage Δ
src/array/list/mod.rs	72.72% <ø> (ø)
src/array/mod.rs	54.62% <ø> (ø)
src/datatypes/mod.rs	71.42% <ø> (ø)
src/ffi/ffi.rs	92.08% <ø> (ø)
src/ffi/mod.rs	100.00% <ø> (ø)
src/io/avro/mod.rs	0.00% <ø> (ø)
src/io/avro/read/mod.rs	77.19% <ø> (ø)
src/io/csv/mod.rs	0.00% <ø> (ø)
src/io/csv/read/deserialize.rs	57.42% <ø> (ø)
src/io/csv/read/reader.rs	63.15% <ø> (ø)
... and 14 more

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