Update documentation in bopomofo module

[wusyong]

Fix #367.

• Removed all TODO and added the missing documents.
• Add trivia to make enum variants document layout slightly better.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.29%. Comparing base (d48ebab) to head (0bf64b3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #582   +/-   ##
=======================================
  Coverage   89.29%   89.29%           
=======================================
  Files          38       38           
  Lines        8221     8221           
=======================================
  Hits         7341     7341           
  Misses        880      880           

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