Update changelog entry for weighted sampling

[victorlin]

Description of proposed changes

Link to the filtering and subsampling guide, which provides in-depth examples on when and how to use the new feature.

Related issue(s)

Follow-up to #1454

Checklist

• [x] Automated checks pass
• [x] Check if you need to add a changelog message
• [x] Check if you need to add tests
• [x] Check if you need to update docs
• [x] Merge+release alongside https://github.com/nextstrain/docs.nextstrain.org/pull/223