Update documentation in bopomofo module

[wusyong]

Fix https://github.com/chewing/libchewing/issues/367.

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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