docs: SSH host verification is done at connect time

[bagder]

Connection reuse is done more "casually" once verified.

[Copilot]

Pull request overview

This PR updates libcurl option documentation to clarify that SSH host verification happens when establishing a new connection, and that subsequent connection reuse may occur without re-running the originally configured verification method.

Changes:

• Document that CURLOPT_SSH_KNOWNHOSTS verification is applied on new connections and that verified connections can later be reused.
• Document similar “verify on connect, then reuse” behavior for CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256 and CURLOPT_SSH_HOST_PUBLIC_KEY_MD5.
• Cross-reference the other SSH host verification options in each doc page to set expectations around connection reuse.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
docs/libcurl/opts/CURLOPT_SSH_KNOWNHOSTS.md	Adds a note describing verification at connect time and that the connection can later be reused.
docs/libcurl/opts/CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256.md	Adds a similar note for SHA256 host key verification and connection reuse behavior.
docs/libcurl/opts/CURLOPT_SSH_HOST_PUBLIC_KEY_MD5.md	Adds a similar note for MD5 host key verification and connection reuse behavior.

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