doc: add note to indicate that the OBJ_ functions were not thread safe in 3.0
doc: add note to indicate that the OBJ_ functions were not thread safe in 3.0

Also remove OBJ_thread from the list of non-threadsafe functions.

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15713)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl