Add doc for ERR_clear_last_mark().

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15258)

doc/man3/ERR_set_mark.pod

@@ -2,15 +2,16 @@
 
 =head1 NAME
 
-ERR_set_mark, ERR_pop_to_mark - set marks and pop errors until mark
+ERR_set_mark, ERR_clear_last_mark, ERR_pop_to_mark
+- set mark, clear mark and pop errors until mark
 
 =head1 SYNOPSIS
 
  #include <openssl/err.h>
 
  int ERR_set_mark(void);
-
  int ERR_pop_to_mark(void);
+ int ERR_clear_last_mark(void);
 
 =head1 DESCRIPTION
 
@@ -20,16 +21,18 @@ is one.
 ERR_pop_to_mark() will pop the top of the error stack until a mark is found.
 The mark is then removed.  If there is no mark, the whole stack is removed.
 
+ERR_clear_last_mark() removes the last mark added if there is one.
+
 =head1 RETURN VALUES
 
 ERR_set_mark() returns 0 if the error stack is empty, otherwise 1.
 
-ERR_pop_to_mark() returns 0 if there was no mark in the error stack, which
-implies that the stack became empty, otherwise 1.
+ERR_clear_last_mark() and ERR_pop_to_mark() return 0 if there was no mark in the
+error stack, which implies that the stack became empty, otherwise 1.
 
 =head1 COPYRIGHT
 
-Copyright 2003-2017 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2003-2021 The OpenSSL Project Authors. All Rights Reserved.
 
 Licensed under the Apache License 2.0 (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy

util/missingcrypto.txt

@@ -596,7 +596,6 @@ EVP_SIGNATURE-ECDSA(7)
 EVP_SIGNATURE-ED25519(7)
 EVP_SIGNATURE-ED448(7)
 EVP_SIGNATURE-RSA(7)
-ERR_clear_last_mark(3)
 ERR_get_state(3)
 ERR_load_ASN1_strings(3)
 ERR_load_ASYNC_strings(3)