CURLMOPT_SOCKETFUNCTION.md: this sends *all* file descriptors

[bagder]

Since libcurl has more than just the main transfer sockets to worry about.

[Copilot]

Pull request overview

Updates the CURLMOPT_SOCKETFUNCTION(3) documentation to clarify that the socket callback can be invoked for more than just “main transfer” sockets, including additional internal file descriptors libcurl uses.

Changes:

• Reformat the SYNOPSIS prototype for readability (argument names and line wrapping).
• Clarify callback behavior/wording and emphasize that libcurl may pass internal FDs (resolver, Happy Eyeballs, etc.) to the socket callback.
• Expand CURL_POLL_REMOVE semantics to note it may not imply immediate close, and mention the curl_multi_assign(3) association is dropped when a socket is removed.

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[testclutch]

Analysis of PR #22081 at a1e099eb:

Test 1275 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 382 different CI jobs (the link just goes to one of them).

Test 3017 failed, but it has been 60.7% flaky lately, so it's probably NOT a fault of the PR. Note that this test has failed in 4 different CI jobs (the link just goes to one of them). Note that this CI job has had a number of other flaky tests recently (2, to be specific) so it may be that this failure is rather a systemic issue with this job and not with this specific PR.

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