[Sync-En] cURL: document multi-handle behavior, update curl-multi-exec purpose, document socket_set_timeout deprecation

[lacatoire]

Closes #3331
Closes #3332
Closes #3337

curl_errno / curl_error — document behavior with multi handles (Closes #3337)

Sync with php/doc-en@ff7a005

• Add note: curl_errno() always returns 0 when handle was executed via curl_multi_exec(); use curl_multi_info_read() instead
• Add note: curl_error() always returns '' when handle was executed via curl_multi_exec(); pass result key from curl_multi_info_read() to curl_strerror()
• Add curl_multi_info_read / curl_strerror to seealso

curl_multi_exec — change function purpose (Closes #3332)

Sync with php/doc-en@d6aa790

• Update refpurpose to "Traite chaque handle dans la file"
• Change <para> to <simpara> for description and return values

socket_set_timeout — document deprecation in PHP 8.5 (Closes #3331)

Sync with php/doc-en@13682c8

• Add <refsynopsisdiv> with &warn.deprecated.function-8-5-0;
• Add seealso section with stream_set_timeout