Result doc comments

[zslayton]

Adds doc comments to:

• The decimal module
• The decimal::coefficient module
• The result module and its nested error types

Fixes #587.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d35d6ae) 82.39% compared to head (49ca3e4) 82.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #605   +/-   ##
=======================================
  Coverage   82.39%   82.39%           
=======================================
  Files         111      111           
  Lines       20048    20048           
  Branches    20048    20048           
=======================================
  Hits        16518    16518           
  Misses       1912     1912           
  Partials     1618     1618           

Impacted Files	Coverage Δ
src/result/decoding_error.rs	83.33% <ø> (ø)
src/result/encoding_error.rs	83.33% <ø> (ø)
src/result/illegal_operation.rs	88.88% <ø> (ø)
src/result/incomplete.rs	90.00% <ø> (ø)
src/result/io_error.rs	0.00% <ø> (ø)
src/result/mod.rs	64.86% <ø> (ø)
src/types/decimal/coefficient.rs	89.09% <ø> (ø)
src/types/decimal/mod.rs	91.09% <ø> (ø)

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