Fix and document BIO_FLAGS_NONCLEAR_RST behavior on memory BIO

The BIO_FLAGS_NONCLEAR_RST flag behavior was not properly documented
and it also caused the length to be incorrectly set after the reset
operation.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9179)

(cherry picked from commit 8b7b32921e63c492fa7233d81b11ee4d7ba266de)