man: document fido_cred_x5c_chain_{ptr,len}

man/fido_cred_new.3

@@ -103,6 +103,8 @@
 .Ft const unsigned char *
 .Fn fido_cred_user_id_ptr "const fido_cred_t *cred"
 .Ft const unsigned char *
+.Fn fido_cred_x5c_chain_ptr "const fido_cred_t *cred" "size_t idx"
+.Ft const unsigned char *
 .Fn fido_cred_x5c_ptr "const fido_cred_t *cred"
 .Ft const unsigned char *
 .Fn fido_cred_attstmt_ptr "const fido_cred_t *cred"
@@ -125,6 +127,8 @@
 .Ft size_t
 .Fn fido_cred_user_id_len "const fido_cred_t *cred"
 .Ft size_t
+.Fn fido_cred_x5c_chain_len "const fido_cred_t *cred" "size_t idx"
+.Ft size_t
 .Fn fido_cred_x5c_len "const fido_cred_t *cred"
 .Ft size_t
 .Fn fido_cred_attstmt_len "const fido_cred_t *cred"
@@ -265,6 +269,18 @@ The corresponding length can be obtained by
 and
 .Fn fido_cred_attstmt_len .
 .Pp
+The
+.Fn fido_cred_x5c_chain_ptr
+function returns a pointer to the
+.Fa idx
+certificate of the chain returned as part of the attestation statement.
+The
+.Fn fido_cred_x5c_chain_len
+function returns the corresponding length.
+Please note that the leaf certificate has a
+.Fa idx
+(index) value of 0.
+.Pp
 The authenticator data, x509 certificate, and signature parts of a
 credential are typically passed to a FIDO2 server for verification.
 .Pp