Make deprecation of ASN1_STRING_data(3) more prominent

The function has been deprecated for a long time: OpenSSL did so when
adding ASN1_STRING_get0_data in 1.1.0. beck removed it from OpenSSL 4.
The reason is that it returns a modifiable "string" and you should not
be modifying an ASN1_STRING this way. You should be using one of the
clumsy, inconvenient accessors.

schwarze merged the deprecation when adding documentation for
ASN1_STRING_get0_data() in 2018. Make this a bit more obvious: move
function to the end of the documented functions in this page, and mark
it deprecated in the SYNOPSIS, too.

I'm aware that deprecation does not work. I was hopeful to be able to
remove the function in the last cycle, but that did not happen and it
might not happen in this cycle either. It will happen at some point,
though.

Prompted by a question by brad

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