Clarify the documentation on the use of ChaCha20

Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8780)

(cherry picked from commit 6f3aae256d62cfcc48c07cc0ead5080b070f371b)

doc/man3/EVP_chacha20.pod

@@ -21,7 +21,15 @@ The ChaCha20 stream cipher for EVP.
 
 =item EVP_chacha20()
 
-The ChaCha20 stream cipher. The key length is 256 bits, the IV is 96 bits long.
+The ChaCha20 stream cipher. The key length is 256 bits, the IV is 128 bits long.
+The first 32 bits consists of a counter in little-endian order followed by a 96
+bit nonce. For example a nonce of:
+
+000000000000000000000002
+
+With an initial counter of 42 (2a in hex) would be expressed as:
+
+2a000000000000000000000000000002
 
 =item EVP_chacha20_poly1305()