Add more commentary about recursive Provider intialisation in the FIPS module

In addition this commit ensures that the "provctx" value is defaulted to the current
library context when we are recurively initialising the FIPS provider when already inside
the FIPS module.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8728)