Document that the openssl fipsinstall self test callback may not be used.

Fixes #16260

If the user autoloads a fips module from a config file, then it will run the self tests early (before the self test callback is set),
and they may not get triggered again during the fipsinstall process.
In order for this to happen there must already be a valid fips config file.
As the main purpose of the application is to generate the fips config file, this case has just been documented.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16475)
