doc: add documentation for BIO_nread0, BIO_nread, BIO_nwrite0, BIO_nwrite

doc: add documentation for BIO_nread0, BIO_nread, BIO_nwrite0, BIO_nwrite

Document the non-copying interface functions for BIO pairs that allow
direct access to the internal buffer:
- BIO_nread0/BIO_nread for zero-copy reading
- BIO_nwrite0/BIO_nwrite for zero-copy writing

Also add return value documentation for these functions and remove
the placeholder "[XXXXX: More return values need to be added here]".

Fixes #16366

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
MergeDate: Wed May  6 14:51:27 2026
(Merged from https://github.com/openssl/openssl/pull/29849)