Document VirtualServer routeSelector no-match warning (NIC)

[promptless]

Proposed changes

Documents nginx/kubernetes-ingress#10913 (which fixes nginx/kubernetes-ingress#10899): F5 NGINX Ingress Controller now emits a Warning event and sets a VirtualServer to State: Warning when its routeSelector matches no VirtualServerRoutes. Previously such a VirtualServer stayed State: Valid with no event, so it silently served no routes for the selected paths — indistinguishable from a healthy VirtualServer. This aligns selector-based attachment with named route: attachment, which already warned.

What changed:

• content/nic/troubleshooting/troubleshoot-virtualserver.md: adds a "Warning events" subsection showing the Warning / AddedOrUpdatedWithWarning event from kubectl describe vs, the conditions that cause it (a selected VirtualServerRoute is deleted, its ingressClassName changes, or its labels change so they no longer match the selector), and how to clear it.
• content/nic/configuration/virtualserver-and-virtualserverroute-resources.md: adds a note in the RouteSelector reference section cross-linking to the new troubleshooting content.

Notes for the reviewer:

• Source PR Warn when routeSelector matches no VirtualServerRoutes kubernetes-ingress#10913 is open and unmerged, so this behavior is not yet in a released version. Please hold or retarget this docs PR so it merges only when the source change ships, to avoid publishing unreleased behavior on the continuously deployed site. The base is main because no NGINX Ingress Controller release branch exists and the source PR carries no milestone or version; retarget it if a release branch is cut.
• This change looks release-note worthy, because it changes the reported state of existing users' VirtualServers. The release note and changelog entry are left to the maintainers per repository convention and are not included here.
• The source PR is still under review with open design questions (whether removing a label as a deliberate detach should warn, and the volume of transient warnings during startup or rollout), so the documented behavior may change before it merges.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 Technical Writing Style Guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

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