Update ChaCha20-Poly1305 documentation

Correctly describe the maximum IV length.
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8406)

(cherry picked from commit 27d5631236325c3fd8a3bd06af282ac496aac64b)

doc/man3/EVP_EncryptInit.pod

@@ -436,7 +436,9 @@ The following I<ctrl>s are supported for the ChaCha20-Poly1305 AEAD algorithm.
 
 Sets the nonce length. This call can only be made before specifying the nonce.
 If not called a default nonce length of 12 (i.e. 96 bits) is used. The maximum
-nonce length is 16 (B<CHACHA_CTR_SIZE>, i.e. 128-bits).
+nonce length is 12 bytes (i.e. 96-bits). If a nonce of less than 12 bytes is set
+then the nonce is automatically padded with leading 0 bytes to make it 12 bytes
+in length.
 
 =item EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_GET_TAG, taglen, tag)