SSL_add_dir_cert_subjects_to_stack(): Documented return values

In the man page for SSL_add_dir_cert_subjects_to_stack(), the functions
returning int have undocumented return values.

Fixes #23171

Signed-off-by: Shakti Shah <shaktishah33@gmail.com>

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #23433)

(cherry picked from commit 9f3a7ca)

doc/man3/SSL_load_client_CA_file.pod

@@ -54,7 +54,8 @@ it is not limited to CA certificates.
 
 =head1 RETURN VALUES
 
-The following return values can occur:
+The following return values can occur for SSL_load_client_CA_file_ex(), and
+SSL_load_client_CA_file():
 
 =over 4
 
@@ -68,6 +69,21 @@ Pointer to the subject names of the successfully read certificates.
 
 =back
 
+The following return values can occur for SSL_add_file_cert_subjects_to_stack(),
+SSL_add_dir_cert_subjects_to_stack(), and SSL_add_store_cert_subjects_to_stack():
+
+=over 4
+
+=item 0 (Failure)
+
+The operation failed.
+
+=item 1 (Success)
+
+The operation succeeded.
+
+=back
+
 =head1 EXAMPLES
 
 Load names of CAs from file and use it as a client CA list: