Clarify NegotiateAuthentication completion semantics

[Copilot]

Ports the documentation improvements from dotnet/dotnet-api-docs#12724 into NegotiateAuthentication XML comments so they appear in IntelliSense.

• Clarifies that IsAuthenticated indicates exchange completion, not success.
• Directs callers to the latest GetOutgoingBlob status code.
• Removes the misleading IsAuthenticated reference from TargetName.

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[azure-pipelines]

Azure Pipelines:
Successfully started running 4 pipeline(s).
12 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

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[Copilot]

🟡 Changes recommended

The ContinueNeeded status needs to be described as an in-progress exchange.

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Pull request overview

This PR updates NegotiateAuthentication XML documentation to clarify authentication completion and status semantics for IntelliSense.

Changes:

• Clarifies IsAuthenticated behavior.
• Directs callers to GetOutgoingBlob status codes.
• Removes the misleading IsAuthenticated reference from TargetName.

The documentation should distinguish ContinueNeeded as an in-progress exchange rather than implying authentication failure.

File summaries

File	Summary
src/libraries/System.Net.Security/src/System/Net/Security/NegotiateAuthentication.cs	Updates public property documentation.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite