Document signature accessors.

Reviewed-by: NMatt Caswell <matt@openssl.org>

doc/crypto/X509_get0_signature.pod

+=pod
+
+=head1 NAME
+
+X509_get0_signature, X509_get_signature_nid, X509_REQ_get0_signature,
+X509_REQ_get_signature_nid, X509_CRL_get0_signature,
+X509_CRL_get_signature_nid - signature information.
+
+=head1 SYNOPSIS
+
+ #include <openssl/x509.h>
+
+ void X509_get0_signature(ASN1_BIT_STRING **psig, X509_ALGOR **palg,
+                          const X509 *x);
+ int X509_get_signature_nid(const X509 *x);
+
+ void X509_REQ_get0_signature(ASN1_BIT_STRING **psig, X509_ALGOR **palg,
+                              const X509_REQ *crl);
+ int X509_REQ_get_signature_nid(const X509_REQ *crl);
+
+ void X509_CRL_get0_signature(ASN1_BIT_STRING **psig, X509_ALGOR **palg,
+                              const X509_CRL *crl);
+ int X509_CRL_get_signature_nid(const X509_CRL *crl);
+
+=head1 DESCRIPTION
+
+X509_get0_signature() sets B<*psig> to the signature of B<x> and B<*palg>
+to the signature algorithm of B<x>. The values returned are internal
+pointers which B<MUST NOT> be freed up after the call.
+
+X509_get_signature_nid() returns the NID corresponding to the signature
+algorithm of B<x>.
+
+X509_REQ_get0_signature(), X509_REQ_get_signature_nid()
+X509_CRL_get0_signature() and X509_CRL_get_signature_nid() perform the
+same function for certificate requests and CRLs.
+
+=head1 NOTES
+
+These functions provide lower level access to signatures in certificates
+where an application wishes to analyse or generate a signature in a form
+where X509_sign() et al is not appropriate (for example a non standard
+or unsupported format).
+
+=head1 RETURN VALUES
+
+X509_get_signature_nid(), X509_REQ_get_signature_nid() and
+X509_CRL_get_signature_nid() return a NID.
+
+X509_get0_signature(), X509_REQ_get0_signature() and
+X509_CRL_get0_signature() do not return values.
+
+=head1 SEE ALSO
+
+L<d2i_X509(3)>,
+L<ERR_get_error(3)>,
+L<X509_CRL_get0_by_serial(3)>,
+L<X509_get_ext_d2i(3)>,
+L<X509_get_extension_flags(3)>,
+L<X509_get_pubkey(3)>,
+L<X509_get_subject_name(3)>,
+L<X509_get_version(3)>,
+L<X509_NAME_add_entry_by_txt(3)>,
+L<X509_NAME_ENTRY_get_object(3)>,
+L<X509_NAME_get_index_by_NID(3)>,
+L<X509_NAME_print_ex(3)>,
+L<X509_new(3)>,
+L<X509_sign(3)>,
+L<X509V3_get_d2i(3)>,
+L<X509_verify_cert(3)>
+
+=head1 HISTORY
+
+X509_get0_signature() and X509_get_signature_nid() were first added to
+OpenSSL 1.0.2.
+
+X509_REQ_get0_signature(), X509_REQ_get_signature_nid(),
+X509_CRL_get0_signature() and X509_CRL_get_signature_nid() were first added
+to OpenSSL 1.1.0.
+
+=cut