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GH-38683: [Python][Docs] Update docstrings for Time32Type and Time64Type (#39059)

### Rationale for this change

`Time32Type` and `Time64Type` unit docs are not correctly documented.

### What changes are included in this PR?

Update the docstrings for `Time32Type` and `Time64Type` `unit`.
* Closes: #38683

Authored-by: AlenkaF <frim.alenka@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>