Update documentation for the /repository/tree endpoint's 404 error code

Problem

#420865 (closed) introduced a change to the Repository Tree response.

See thread: !173543 (comment 2291512330)

Before

https://gitlab.com/api/v4/projects/278964/repository/tree?ref=master&path=unknown

would return 200 status code and response []

After

https://gitlab.com/api/v4/projects/278964/repository/tree?ref=master&path=unknown

returns 404 error code.

This change affected external integrations that didn't handle a potential error code coming from this endpoint.

Proposal

Update documentation to highlight this change