Document X509 version functions.

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

diff --git a/doc/crypto/X509_get_version.pod b/doc/crypto/X509_get_version.pod
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+=pod
+
+=head1 NAME
+
+X509_get_version, X509_set_version, X509_REQ_get_version, X509_REQ_set_version,
+X509_CRL_get_version, X509_CRL_set_version - get or set certificate,
+certificate request or CRL version.
+
+=head1 SYNOPSIS
+
+ #include <openssl/x509.h>
+
+ long X509_get_version(X509 *x);
+ int X509_set_version(X509 *x, long version);
+
+ long X509_REQ_get_version(X509_REQ *req);
+ int X509_REQ_set_version(X509_REQ *x, long version);
+
+ long X509_CRL_get_version(X509_CRL *crl);
+ int X509_CRL_set_version(X509_CRL *x, long version);
+
+=head1 DESCRIPTION
+
+X509_get_version() returns the numerical value of the version field of
+certificate B<x>. Note: this is defined by standards (X.509 et al) to be one
+less than the certificate version. So a verson 3 certificate will return 2 and
+a version 1 certificate will return 0.
+
+X509_set_version() sets the numerical value of the version field of certificate
+B<x> to B<version>.
+
+Similarly X509_REQ_get_version(), X509_REQ_set_version(),
+X509_CRL_get_version() and X509_CRL_set_version() get and set the version
+number of certificate requests and CRLs.
+
+=head1 NOTES
+
+The version field of certificates, certificate requests and CRLs has a
+DEFAULT value of B<v1(0)> meaning the field should be omitted for version
+1. This is handled transparently by these functions.
+
+=head1 RETURN VALUES
+
+X509_get_version(), X509_REQ_get_version() and X509_CRL_get_version()
+return the numerical value of the version field.
+
+X509_set_version(), X509_REQ_set_version() and X509_CRL_set_version()
+return 1 for success and 0 for failure.
+
+=head1 SEE ALSO
+
+L<d2i_X509(3)>,
+L<ERR_get_error(3)>,
+L<X509_CRL_get0_by_serial(3)>,
+L<X509_get0_signature(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_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_get_version(), X509_set_version() X509_REQ_get_version(),
+X509_REQ_set_version(), X509_REQ_get_version() and X509_REQ_set_version()
+are available in all versions of OpenSSL.
+
+X509_get_version(), X509_REQ_get_version() and X509_CRL_get_version() are
+functions in OpenSSL 1.1.0, in previous versions they were macros.
+
+=cut