[DOC] Clarify some details about `where` and `where_document`

[hesreallyhim]

Description of changes

Following up on #4280

Adds some details about the filters, making clear that:

• filter dictionaries must have one key
• $in/$nin lists must contain one or more values of the same type.

Summarize the changes made by this PR.

• Improvements & Bug fixes
  • ...
• New functionality
  • ...

Test plan

How are these changes tested?

• [ ] Tests pass locally with pytest for python, yarn test for js, cargo test for rust

Documentation Changes

Are all docstrings for user-facing APIs updated if required? Do we need to make documentation changes in the docs section?