Document pkcs12 -password behavior

apps/pkcs12.c accepts -password as an argument.  The document author
almost certainly meant to write "-password, -passin".

However, that is not correct, either.  Actually the code treats
-password as equivalent to -passin, EXCEPT when -export is also
specified, in which case -password as equivalent to -passout.
(cherry picked from commit 856c6dfb)