Document the deprecation of ERR_func_error_string()

Also, correct the output template for ERR_error_string() and
ERR_error_string_n().

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9756)

doc/man3/ERR_error_string.pod

@@ -14,9 +14,12 @@ error message
  void ERR_error_string_n(unsigned long e, char *buf, size_t len);
 
  const char *ERR_lib_error_string(unsigned long e);
- const char *ERR_func_error_string(unsigned long e);
  const char *ERR_reason_error_string(unsigned long e);
 
+Deprecated in OpenSSL 3.0:
+
+ const char *ERR_func_error_string(unsigned long e);
+
 =head1 DESCRIPTION
 
 ERR_error_string() generates a human-readable string representing the
@@ -33,13 +36,12 @@ For ERR_error_string_n(), I<buf> may not be B<NULL>.
 
 The string will have the following format:
 
- error:[error code]:[library name]:[function name]:[reason string]
+ error:[error code]:[library name]::[reason string]
 
-I<error code> is an 8 digit hexadecimal number, I<library name>,
-I<function name> and I<reason string> are ASCII text.
+I<error code> is an 8 digit hexadecimal number, I<library name> and
+I<reason string> are ASCII text.
 
-ERR_lib_error_string(), ERR_func_error_string() and
-ERR_reason_error_string() return the library name, function
+ERR_lib_error_string() and ERR_reason_error_string() return the library
 name and reason string respectively.
 
 If there is no text string registered for the given error code,
@@ -53,15 +55,20 @@ all error codes currently in the queue.
 ERR_error_string() returns a pointer to a static buffer containing the
 string if I<buf> B<== NULL>, I<buf> otherwise.
 
-ERR_lib_error_string(), ERR_func_error_string() and
-ERR_reason_error_string() return the strings, and B<NULL> if
-none is registered for the error code.
+ERR_lib_error_string() and ERR_reason_error_string() return the strings,
+and B<NULL> if none is registered for the error code.
+
+ERR_func_error_string() returns NULL.
 
 =head1 SEE ALSO
 
 L<ERR_get_error(3)>,
 L<ERR_print_errors(3)>
 
+=head1 HISTORY
+
+ERR_func_error_string() became deprecated in OpenSSL 3.0.
+
 =head1 COPYRIGHT
 
 Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.