Improve API product model field descriptions

[CaralHsi]

Some API product models have incomplete field descriptions. This issue is to improve those descriptions so generated API documentation is clearer.

Target file

• src/memos/api/product_models.py

Existing TODO

# TODO: maybe add detailed description later

Requirements

• Add clear and concise descriptions for fields that currently lack detail.
• Keep model behavior unchanged.
• Do not rename fields or change validation logic.
• Follow the style of existing descriptions in the file.

Acceptance Criteria

• The TODO is resolved.
• API model descriptions are clearer.
• No behavior changes are introduced.