Clear up documentation of ENGINES

[jogme]

The documentation should be updated stating the removed functions.

[mbroz]

Also documentation should be revised to no longer contain mention of removed man pages and engines description in general.

[mbroz]

A per openssl/openssl#28548 (comment) comment, the man page should stay, should reference all legacy APIs (no need to use missing wrapper) but without full description and should reference OPENSSL_ENGINE_STUB macro.

[mbroz]

OPENSSL_ENGINE_STUB doc should probably go into openssl_user_macros(7) man page.