first commit, SM2-DSA only

doc/crypto/BIO_f_null.pod

@@ -0,0 +1,32 @@
+=pod
+
+=head1 NAME
+
+BIO_f_null - null filter
+
+=head1 SYNOPSIS
+
+ #include <openssl/bio.h>
+
+ BIO_METHOD *	BIO_f_null(void);
+
+=head1 DESCRIPTION
+
+BIO_f_null() returns the null filter BIO method. This is a filter BIO
+that does nothing.
+
+All requests to a null filter BIO are passed through to the next BIO in
+the chain: this means that a BIO chain containing a null filter BIO
+behaves just as though the BIO was not there.
+
+=head1 NOTES
+
+As may be apparent a null filter BIO is not particularly useful.
+
+=head1 RETURN VALUES
+
+BIO_f_null() returns the null filter BIO method.
+
+=head1 SEE ALSO
+
+TBA