[PATCH] Clarify that ssl_groups is for any key exchange groups

Hi,

The ssl_groups parameter introduced in Postgres 18 decided to use a short_desc: "Sets the group(s) to use for Diffie-Hellman key exchange" [1]. The documentation still references curves [2].

However, this parameter is just passed through to SSL_CTX_set1_groups_list. This means the parameter readily accepts values like a pure `MLKEM768`, assuming the crypto lib supports it, which is true since OpenSSL 3.5. Yet these Shor-safe groups are not DH key exchange.

I think it makes sense to modify the documentation to a more generic one to reflect the capabilities of ssl_groups more accurately, e.g. "Sets the named groups to use for TLS key exchange."

A more concrete patch suggestion is attached.

Evan

[1] https://www.postgresql.org/message-id/D44791DD-0CD9-48A7-9471-60593673A91B%40yesql.se
[2] https://www.postgresql.org/docs/18/runtime-config-connection.html#GUC-SSL-GROUPS