Align InitializeResult.instructions XML docs with latest MCP spec guidance

[Copilot]

This updates SDK XML documentation to reflect the MCP spec guidance introduced in modelcontextprotocol/modelcontextprotocol@ff8bd88: initialize instructions should help models use the server effectively and should not duplicate primitive descriptions.

• Protocol DTO docs (InitializeResult)

  • Updated InitializeResult.Instructions remarks to emphasize model-usage guidance (workflow/capability conventions) over restating primitive metadata.
  • Explicitly calls out avoiding duplication of tool/prompt/resource descriptions already available via protocol responses.

• Client API docs (McpClient.ServerInstructions)

  • Aligned client-facing wording with spec intent: treat instructions as high-level usage guidance, not repeated primitive docs.

• Server options docs (McpServerOptions.ServerInstructions)

  • Aligned server-side configuration remarks to guide authors toward non-duplicative, model-oriented instruction content.

Example of the updated guidance style:

/// They should focus on information that helps models use the server effectively
/// and should not duplicate tool, prompt, or resource descriptions already exposed elsewhere.
public string? ServerInstructions { get; set; }

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