doc/man3/SSL_set_fd.pod: add note about Windows compiler warning

According to an old stackoverflow thread [1], citing an even older comment by
Andy Polyakov (1875e6db29, Pull up Win64 support from 0.9.8., 2005-07-05),
a cast of 'SOCKET' (UINT_PTR) to 'int' does not create a problem, because although
the documentation [2] claims that the upper limit is INVALID_SOCKET-1 (2^64 - 2),
in practice the socket() implementation on Windows returns an index into the kernel
handle table, the size of which is limited to 2^24 [3].

Add this note to the manual page to avoid unnecessary roundtrips to StackOverflow.

[1] https://stackoverflow.com/questions/1953639/is-it-safe-to-cast-socket-to-int-under-win64
[2] https://docs.microsoft.com/en-us/windows/win32/winsock/socket-data-type-2
[3] https://docs.microsoft.com/en-us/windows/win32/sysinfo/kernel-objects

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16699)

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