Document EC_curve_nist2nid and EC_curve_nid2nist functions

Document EC_curve_nist2nid and EC_curve_nid2nist functions

Add documentation for EC_curve_nist2nid() and EC_curve_nid2nist()
functions which were previously undocumented. These functions convert
between OpenSSL NIDs and NIST standardized curve names (e.g., "P-256",
"P-384", "P-521" for prime field curves and "B-163", "K-233", etc.
for binary field curves).

The documentation includes:
 - Function prototypes in the SYNOPSIS section
 - Detailed descriptions explaining the purpose and behavior
 - Examples of NIST curve names
 - Return value documentation

Fixes #29180

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
MergeDate: Mon Feb  9 09:59:04 2026
(Merged from https://github.com/openssl/openssl/pull/29341)