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.

doc/apps/pkcs12.pod

@@ -67,7 +67,7 @@ by default.
 The filename to write certificates and private keys to, standard output by
 default.  They are all written in PEM format.
 
-=item B<-pass arg>, B<-passin arg>
+=item B<-passin arg>
 
 the PKCS#12 file (i.e. input file) password source. For more information about
 the format of B<arg> see the B<PASS PHRASE ARGUMENTS> section in
@@ -79,6 +79,11 @@ pass phrase source to encrypt any outputed private keys with. For more
 information about the format of B<arg> see the B<PASS PHRASE ARGUMENTS> section
 in L<openssl(1)|openssl(1)>.
 
+=item B<-password arg>
+
+With -export, -password is equivalent to -passout.
+Otherwise, -password is equivalent to -passin.
+
 =item B<-noout>
 
 this option inhibits output of the keys and certificates to the output file