Add manpage for EVP_PKEY_get_field_type and EVP_PKEY_get_point_conv_form

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13139)

doc/man3/EVP_PKEY_get_field_type.pod

@@ -0,0 +1,55 @@
+=pod
+
+=head1 NAME
+
+EVP_PKEY_get_field_type, EVP_PKEY_get_ec_point_conv_form - get field type
+or point conversion form of a key
+
+=head1 SYNOPSIS
+
+ #include <openssl/evp.h>
+
+ int EVP_PKEY_get_field_type(const EVP_PKEY *pkey);
+ int EVP_PKEY_get_ec_point_conv_form(const EVP_PKEY *pkey);
+
+=head1 DESCRIPTION
+
+EVP_PKEY_get_field_type() returns the field type NID of the I<pkey>, if
+I<pkey>'s key type supports it. The types currently supported
+by the built-in OpenSSL providers are either B<NID_X9_62_prime_field>
+for prime curves or B<NID_X9_62_characteristic_two_field> for binary curves;
+these values are defined in the F<< <openssl/obj_mac.h> >> header file.
+
+EVP_PKEY_get_ec_point_conv_form() returns the point conversion format
+of the I<pkey>, if I<pkey>'s key type supports it.
+
+=head1 NOTES
+
+Among the standard OpenSSL key types, this is only supported for EC and
+SM2 keys.  Other providers may support this for additional key types.
+
+=head1 RETURN VALUES
+
+EVP_PKEY_get_field_type() returns the field type NID or 0 on error.
+
+EVP_PKEY_get_ec_point_conv_form() returns the point conversion format number
+(see L<EC_GROUP_copy(3)>) or 0 on error.
+
+=head1 SEE ALSO
+
+L<EC_GROUP_copy(3)>
+
+=head1 HISTORY
+
+These functions were added in OpenSSL 3.0.
+
+=head1 COPYRIGHT
+
+Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the Apache License 2.0 (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut