Document more EVP_MD_CTX functions

Fixes #6644

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6645)

doc/man3/EVP_DigestInit.pod

@@ -3,11 +3,13 @@
 =head1 NAME
 
 EVP_MD_CTX_new, EVP_MD_CTX_reset, EVP_MD_CTX_free, EVP_MD_CTX_copy_ex,
-EVP_MD_CTX_ctrl, EVP_DigestInit_ex, EVP_DigestInit, EVP_DigestUpdate,
+EVP_MD_CTX_ctrl, EVP_MD_CTX_set_flags, EVP_MD_CTX_clear_flags,
+EVP_MD_CTX_test_flags, EVP_DigestInit_ex, EVP_DigestInit, EVP_DigestUpdate,
 EVP_DigestFinal_ex, EVP_DigestFinalXOF, EVP_DigestFinal,
 EVP_MD_CTX_copy, EVP_MD_type, EVP_MD_pkey_type, EVP_MD_size,
 EVP_MD_block_size, EVP_MD_CTX_md, EVP_MD_CTX_size,
-EVP_MD_CTX_block_size, EVP_MD_CTX_type, EVP_md_null,
+EVP_MD_CTX_block_size, EVP_MD_CTX_type, EVP_MD_CTX_md_data,
+EVP_md_null,
 EVP_get_digestbyname, EVP_get_digestbynid,
 EVP_get_digestbyobj - EVP digest routines
 
@@ -19,6 +21,9 @@ EVP_get_digestbyobj - EVP digest routines
  int EVP_MD_CTX_reset(EVP_MD_CTX *ctx);
  void EVP_MD_CTX_free(EVP_MD_CTX *ctx);
  void EVP_MD_CTX_ctrl(EVP_MD_CTX *ctx, int cmd, int p1, void* p2);
+ void EVP_MD_CTX_set_flags(EVP_MD_CTX *ctx, int flags);
+ void EVP_MD_CTX_clear_flags(EVP_MD_CTX *ctx, int flags);
+ int EVP_MD_CTX_test_flags(const EVP_MD_CTX *ctx, int flags);
 
  int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
  int EVP_DigestUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
@@ -41,6 +46,7 @@ EVP_get_digestbyobj - EVP digest routines
  int EVP_MD_CTX_size(const EVP_MD *ctx);
  int EVP_MD_CTX_block_size(const EVP_MD *ctx);
  int EVP_MD_CTX_type(const EVP_MD *ctx);
+ void *EVP_MD_CTX_md_data(const EVP_MD_CTX *ctx);
 
  const EVP_MD *EVP_md_null(void);
 
@@ -72,6 +78,10 @@ Cleans up digest context B<ctx> and frees up the space allocated to it.
 
 Performs digest-specific control actions on context B<ctx>.
 
+=item EVP_MD_CTX_set_flags(), EVP_MD_CTX_clear_flags(), EVP_MD_CTX_test_flags()
+
+Sets, clears and tests B<ctx> flags.  See L</FLAGS> below for more information.
+
 =item EVP_DigestInit_ex()
 
 Sets up digest context B<ctx> to use a digest B<type> from ENGINE B<impl>.
@@ -140,6 +150,12 @@ Return the NID of the OBJECT IDENTIFIER representing the given message digest
 when passed an B<EVP_MD> structure.  For example, C<EVP_MD_type(EVP_sha1())>
 returns B<NID_sha1>. This function is normally used when setting ASN1 OIDs.
 
+=item EVP_MD_CTX_md_data()
+
+Return the digest method private data for the passed B<EVP_MD_CTX>.
+The space is allocated by OpenSSL and has the size originally set with
+EVP_MD_meth_set_app_datasize().
+
 =item EVP_MD_CTX_md()
 
 Returns the B<EVP_MD> structure corresponding to the passed B<EVP_MD_CTX>.
@@ -165,6 +181,38 @@ B<ASN1_OBJECT> structure respectively.
 
 =back
 
+=head1 FLAGS
+
+EVP_MD_CTX_set_flags(), EVP_MD_CTX_clear_flags() and EVP_MD_CTX_test_flags()
+can be used the manipulate and test these B<EVP_MD_CTX> flags:
+
+=over 4
+
+=item EVP_MD_CTX_FLAG_ONESHOT
+
+This flag instructs the digest to optimize for one update only, if possible.
+
+=for comment EVP_MD_CTX_FLAG_CLEANED is internal, don't mention it
+
+=for comment EVP_MD_CTX_FLAG_REUSE is internal, don't mention it
+
+=for comment We currently avoid documenting flags that are only bit holder:
+EVP_MD_CTX_FLAG_NON_FIPS_ALLOW, EVP_MD_CTX_FLAGS_PAD_*
+
+=item EVP_MD_CTX_FLAG_NO_INIT
+
+This flag instructs EVP_DigestInit() and similar not to initialise the
+implementation specific data.
+
+=item EVP_MD_CTX_FLAG_FINALISE
+
+Some functions such as EVP_DigestSign only finalise copies of internal
+contexts so additional data can be included after the finalisation call.
+This is inefficient if this functionality is not required, and can be
+disabled with this flag.
+
+=back
+
 =head1 RETURN VALUES
 
 =over 4