docs/INTERNALS: follow up fix for 11e8066e and 92b9ae5c

 in future libcurl versions.

 To deal with this internally in the best way possible, we have a generic SSL

 function API as provided by the sslgen.[ch] system, and they are the only SSL

 functions we must use from within libcurl. sslgen is then crafted to use the

 function API as provided by the vtls.[ch] system, and they are the only SSL

 functions we must use from within libcurl. vtls is then crafted to use the

 appropriate lower-level function calls to whatever SSL library that is in

 use.

 use. For example vtls/openssl.[ch] for the OpenSSL library.

Library Symbols

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