doc: Clarify that BN_CTX must not be NULL for BN arithmetic functions

doc: Clarify that BN_CTX must not be NULL for BN arithmetic functions

The documentation for BN_add and related functions did not explicitly
state that the ctx parameter cannot be NULL. Users may assume NULL is
acceptable since some other OpenSSL functions allow it, but passing
NULL to functions like BN_mod_add() or BN_mod() causes a crash.

Update the documentation to explicitly state that ctx must not be NULL.

Fixes #12092

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
MergeDate: Wed Apr 15 11:47:59 2026
(Merged from https://github.com/openssl/openssl/pull/29773)