Document the HMAC_size() function

Reviewed-by: NRich Salz <rsalz@openssl.org>

doc/man3/HMAC.pod

@@ -12,7 +12,8 @@ HMAC_Update,
 HMAC_Final,
 HMAC_CTX_copy,
 HMAC_CTX_set_flags,
-HMAC_CTX_get_md
+HMAC_CTX_get_md,
+HMAC_size
 - HMAC message authentication code
 
 =head1 SYNOPSIS
@@ -37,6 +38,8 @@ HMAC_CTX_get_md
  void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
  const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);
 
+ size_t HMAC_size(const HMAC_CTX *e);
+
 Deprecated:
 
  #if OPENSSL_API_COMPAT < 0x10100000L
@@ -105,6 +108,8 @@ These flags have the same meaning as for L<EVP_MD_CTX_set_flags(3)>.
 HMAC_CTX_get_md() returns the EVP_MD that has previously been set for the
 supplied HMAC_CTX.
 
+HMAC_size() returns the length in bytes of the underlying hash function output.
+
 =head1 RETURN VALUES
 
 HMAC() returns a pointer to the message authentication code or NULL if
@@ -119,6 +124,9 @@ HMAC_CTX_copy() return 1 for success or 0 if an error occurred.
 HMAC_CTX_get_md() return the EVP_MD previously set for the supplied HMAC_CTX or
 NULL if no EVP_MD has been set.
 
+HMAC_size() returns the length in bytes of the underlying hash function output
+or zero on error.
+
 =head1 CONFORMING TO
 
 RFC 2104