Update the EVP_PKEY_get_id documentation

The documentation didn't mention the development where EVP_PKEY_get_id()
returns a negative value for provider-only implementations, and the
migration guide didn't mention how to cope with that.

Fixes #20497

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20501)

(cherry picked from commit a2a543e0)