Clarify the usage of EVP_PKEY_get_raw_[private|public]_key()

EVP_PKEY_get_raw_private_key() and EVP_PKEY_get_raw_public_key() expect
the size of the key buffer to be populated in the |*len| parameter on
entry - but the docs made no mention of this.

Fixes #11245
Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11254)

(cherry picked from commit f529fc7d53bf4228fae61cb1efe73d97fe3eb35f)

doc/man3/EVP_PKEY_new.pod

@@ -72,21 +72,24 @@ EVP_PKEY_new_mac_key() works in the same way as EVP_PKEY_new_raw_private_key().
 New applications should use EVP_PKEY_new_raw_private_key() instead.
 
 EVP_PKEY_get_raw_private_key() fills the buffer provided by B<priv> with raw
-private key data. The number of bytes written is populated in B<*len>. If the
-buffer B<priv> is NULL then B<*len> is populated with the number of bytes
-required to hold the key. The calling application is responsible for ensuring
-that the buffer is large enough to receive the private key data. This function
-only works for algorithms that support raw private keys. Currently this is:
-B<EVP_PKEY_HMAC>, B<EVP_PKEY_POLY1305>, B<EVP_PKEY_SIPHASH>, B<EVP_PKEY_X25519>,
-B<EVP_PKEY_ED25519>, B<EVP_PKEY_X448> or B<EVP_PKEY_ED448>.
+private key data. The size of the B<priv> buffer should be in B<*len> on entry
+to the function, and on exit B<*len> is updated with the number of bytes
+actually written. If the buffer B<priv> is NULL then B<*len> is populated with
+the number of bytes required to hold the key. The calling application is
+responsible for ensuring that the buffer is large enough to receive the private
+key data. This function only works for algorithms that support raw private keys.
+Currently this is: B<EVP_PKEY_HMAC>, B<EVP_PKEY_POLY1305>, B<EVP_PKEY_SIPHASH>,
+B<EVP_PKEY_X25519>, B<EVP_PKEY_ED25519>, B<EVP_PKEY_X448> or B<EVP_PKEY_ED448>.
 
 EVP_PKEY_get_raw_public_key() fills the buffer provided by B<pub> with raw
-public key data. The number of bytes written is populated in B<*len>. If the
-buffer B<pub> is NULL then B<*len> is populated with the number of bytes
-required to hold the key. The calling application is responsible for ensuring
-that the buffer is large enough to receive the public key data. This function
-only works for algorithms that support raw public  keys. Currently this is:
-B<EVP_PKEY_X25519>, B<EVP_PKEY_ED25519>, B<EVP_PKEY_X448> or B<EVP_PKEY_ED448>.
+public key data. The size of the B<pub> buffer should be in B<*len> on entry
+to the function, and on exit B<*len> is updated with the number of bytes
+actually written. If the buffer B<pub> is NULL then B<*len> is populated with
+the number of bytes required to hold the key. The calling application is
+responsible for ensuring that the buffer is large enough to receive the public
+key data. This function only works for algorithms that support raw public  keys.
+Currently this is: B<EVP_PKEY_X25519>, B<EVP_PKEY_ED25519>, B<EVP_PKEY_X448> or
+B<EVP_PKEY_ED448>.
 
 =head1 NOTES