Clarify reference count handling/removal of session

during L<SSL_CTX_flush_sessions(3)|SSL_CTX_flush_sessions(3)>.

If the data is to be kept, SSL_get1_session() will increment the reference

count and the session will stay in memory until explicitly freed with

L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>, regardless of its state.

count, so that the session will not be implicitly removed by other operations

but stays in memory. In order to remove the session

L<SSL_SESSION_free(3)|SSL_SESSION_free(3)> must be explicitly called once

to decrement the reference count again.

SSL_SESSION objects keep internal link information about the session cache

list, when being inserted into one SSL_CTX object's session cache.