add description of -attime to man page

the verify app man page didn't describe the usage of attime option
even though it was listed as a valid option in the -help message.

This patch fixes this omission.

doc/apps/verify.pod

@@ -25,6 +25,7 @@ B<openssl> B<verify>
 [B<-untrusted file>]
 [B<-help>]
 [B<-issuer_checks>]
+[B<-attime timestamp>]
 [B<-verbose>]
 [B<->]
 [certificates]
@@ -80,6 +81,12 @@ rejected. The presence of rejection messages does not itself imply that
 anything is wrong; during the normal verification process, several
 rejections may take place.
 
+=item B<-attime timestamp>
+
+Perform validation checks using time specified by B<timestamp> and not
+current system time. B<timestamp> is the number of seconds since
+01.01.1970 (UNIX time).
+
 =item B<-policy arg>
 
 Enable policy processing and add B<arg> to the user-initial-policy-set (see