Add documentation for the BIO_s_mem pecularities

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8649)

(cherry picked from commit 3d42833d389134b7b05b655c264e4dba5a2179e9)