Clarify negative return from X509_verify_cert()

Reviewed-by: NRichard Levitte <levitte@openssl.org>

doc/crypto/X509_verify_cert.pod

@@ -31,12 +31,13 @@ Applications rarely call this function directly but it is used by
 OpenSSL internally for certificate validation, in both the S/MIME and
 SSL/TLS code.
 
-The negative return value from X509_verify_cert() can only occur if no
-certificate is set in B<ctx> (due to a programming error); if X509_verify_cert()
-twice without reinitialising B<ctx> in between; or if a retry
-operation is requested during internal lookups (which never happens with
-standard lookup methods). It is however recommended that application check
-for <= 0 return value on error.
+A negative return value from X509_verify_cert() can occur if it is invoked
+incurrectly, such as with no certificate set in B<ctx>, or when it is called
+twice in succession without reinitialising B<ctx> for the second call.
+A negative return value can also happen due to internal resource problems or if
+a retry operation is requested during internal lookups (which never happens
+with standard lookup methods).
+Applications must check for <= 0 return value on error.
 
 =head1 BUGS