Clarify HttpClient connection-pool disposal

[Copilot]

HttpClient disposal doesn't always dispose pooled connections. The outcome depends on whether the client owns its underlying handler.

• Connection pool ownership

  • Clarify that disposing an HttpClient disposes pooled connections only when it also disposes its handler.
  • State that clients using externally managed handlers leave the handler and connections intact when disposed.

• Factory-managed clients

  • Explain that IHttpClientFactory manages and reuses handlers and their connection pools.
  • Clarify that a new connection is required only after the handler is eventually disposed.

• Fixes Misleading instruction #47972

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docs/fundamentals/networking/http/httpclient-guidelines.md	docs/fundamentals/networking/http/httpclient-guidelines

[Copilot]

Copilot wasn't able to review any files in this pull request.

[MihaZupan]

Thanks