Document the bizarre fact that {CMS,PCKS7}_get0_signers() needs some

Document the bizarre fact that {CMS,PCKS7}_get0_signers() needs some
freeing of what they return despite being get0 functions: the stack
of X509s that they return must be freed with sk_X509_free(). The get0
thus probably refers to the individual certs, but not to the stack
itself.

The libcrypto and libssl APIs never cease to amaze with new traps.

ok inoguchi