Document SSL_R_UNEXPECTED_EOF_WHILE_READING

Also document that it is ok to use this for control flow decisions.

doc/man3/SSL_get_error.pod

@@ -32,7 +32,8 @@ Some TLS implementations do not send a close_notify alert on shutdown.
 On an unexpected EOF, versions before OpenSSL 3.0 returned
 B<SSL_ERROR_SYSCALL>, nothing was added to the error stack, and errno was 0.
 Since OpenSSL 3.0 the returned error is B<SSL_ERROR_SSL> with a meaningful
-error on the error stack.
+error on the error stack (SSL_R_UNEXPECTED_EOF_WHILE_READING). This error reason
+code may be used for control flow decisions.
 
 =head1 RETURN VALUES
 

ssl/record/rec_layer_s3.c

@@ -466,6 +466,10 @@ int ossl_tls_handle_rlayer_return(SSL_CONNECTION *s, int writing, int ret,
             } else {
                 ERR_new();
                 ERR_set_debug(file, line, 0);
+                /*
+                 * This reason code is part of the API and may be used by
+                 * applications for control flow decisions.
+                 */
                 ossl_statem_fatal(s, SSL_AD_DECODE_ERROR,
                                   SSL_R_UNEXPECTED_EOF_WHILE_READING, NULL);
             }