DOCS: add openssl-core_names.h(7)

A CAVEATS section is present in this manual.  That section name is
borrowed from OpenBSD, where mdoc(7) explains it like this:

    CAVEATS
    Common misuses and misunderstandings should be explained in this
    section.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11963)

doc/man7/openssl-core_names.h.pod

@@ -0,0 +1,49 @@
+=pod
+
+=head1 NAME
+
+openssl/core_names.h - OpenSSL provider parameter names
+
+=head1 SYNOPSIS
+
+ #include <openssl/core_names.h>
+
+=head1 DESCRIPTION
+
+The F<< <openssl/core_names.h> >> header defines a multitude of macros
+for L<OSSL_PARAM(3)> names, algorithm names and other known names used
+with OpenSSL's providers, made available for practical purposes only.
+
+Existing names are further described in the manuals for OpenSSL's
+providers (see L</SEE ALSO>) and the manuals for each algorithm they
+provide (listed in those provider manuals).
+
+=head1 SEE ALSO
+
+L<OSSL_PROVIDER-default(7)>, L<OSSL_PROVIDER-FIPS(7)>,
+L<OSSL_PROVIDER-legacy(7)>
+
+=head1 HISTORY
+
+The macros described here were added in OpenSSL 3.0.
+
+=head1 CAVEATS
+
+I<This header file does not constitute a general registry of names>.
+Providers that implement new algorithms are to be responsible for
+their own parameter names.
+
+However, authors of provider that implement their own variants of
+algorithms that OpenSSL providers support will want to pay attention
+to the names provided in this header to work in a compatible manner.
+
+=head1 COPYRIGHT
+
+Copyright 2020 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