XML docs: repeat the cold-VCS caveat on VcsClient.GetVcsAsync #512

The guide's caveat that VCS data can be empty right after start sits inside the model-readiness bullets of getting-started. A builder read the branch at once and got nothing.

Proposed: repeat the caveat in the XML documentation of VcsClient.GetVcsAsync.

Source: internal developer-experience evaluation of the 2.0.32 nightly, 2026-10-18 (finding D32-14).