Add documentation for the EVP_CIPHER_CTX_cipher_data functions

=pod

=head1 NAME

EVP_CIPHER_CTX_cipher_data, EVP_CIPHER_CTX_set_cipher_data - Routines to inspect

and modify EVP_CIPHER_CTX objects

=head1 SYNOPSIS

 #include <openssl/evp.h>

 void *EVP_CIPHER_CTX_cipher_data(const EVP_CIPHER_CTX *ctx);

 void *EVP_CIPHER_CTX_set_cipher_data(EVP_CIPHER_CTX *ctx, void *cipher_data);

=head1 DESCRIPTION

The EVP_CIPHER_CTX_cipher_data() function returns a pointer to the cipher data

relevant to EVP_CIPHER_CTX. The contents of this data is specific to the

particular implementation of the cipher. For example this data can be used by

engines to store engine specific information. The data is automatically

allocated and freed by OpenSSL, so applications and engines should not normally

free this directly (but see below).

The EVP_CIPHER_CTX_set_cipher_data() function allows an application or engine to

replace the cipher data with new data. A pointer to any existing cipher data is

returned from this function. If the old data is no longer required then it

should be freed through a call to OPENSSL_free().

=head1 RETURN VALUES

The EVP_CIPHER_CTX_cipher_data() function returns a pointer to the current

cipher data for the EVP_CIPHER_CTX.

The EVP_CIPHER_CTX_set_cipher_data() function returns a pointer to the old

cipher data for the EVP_CIPHER_CTX.

=head1 HISTORY

The EVP_CIPHER_CTX_set_cipher_data() function was added in OpenSSL 1.1.0.

=cut