Add documentation for the EVP_CIPHER_CTX_cipher_data functions

The new pipeline code added a new function
EVP_CIPHER_CTX_set_cipher_data(). Add documentation for this and the
existing EVP_CIPHER_CTX_cipher_data() function.
Reviewed-by: NTim Hudson <tjh@openssl.org>

doc/crypto/EVP_CIPHER_CTX_cipher_data.pod

+=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