Avoid mentioning uninitialised contexts.

All contexts must be initialised because they can only be created using the _new() calls.
Remove the outdated mentions of uninitialised and initialised contexts.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/5388)