doc: fix RSA_set_method return value documentation

doc: fix RSA_set_method return value documentation

The documentation incorrectly stated that RSA_set_method() returns
a pointer to the old RSA_METHOD. In fact, it returns int (1 for success).
The SYNOPSIS correctly shows 'int RSA_set_method(...)' but the
RETURN VALUES section was wrong.

Fixes #13884

CLA: trivial

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
MergeDate: Thu Feb 12 15:18:38 2026
(Merged from https://github.com/openssl/openssl/pull/29916)