In bio.h rev. 1.54, jsing@ and tb@ provided BIO_callback_fn_ex(3),

In bio.h rev. 1.54, jsing@ and tb@ provided BIO_callback_fn_ex(3),
BIO_set_callback_ex(3), BIO_get_callback_ex(3), and BIO_callback_fn(3).
Document them, in part by merging from the OpenSSL 1.1.1 branch,
which is still under a free license,
but heavily tweaked by me, in particular:
* mention that BIO_set_callback_arg(3) is misnamed;
* keep our more detailed explanation of the "ret" argument;
* make the list of callback invocations more readable;
* and update the HISTORY section.