Documentation fix

Add relevant comments and explanations.
Fix indentation.
Add copyright notes.

programs/util/syslog2stderr.c

@@ -1,3 +1,31 @@
+/** \brief Syslog to stderr wrapper for Unix-like systems
+ *
+ * By dynamically linking this module into an executable, any message sent to the system logs
+ * via the POSIX or Linux API is instead redirected to standard error.
+*
+* Compile this program with `cc -fPID -shared -o syslog2stderr.so syslog2stderr.c -ldl`
+* and load it dynamically when running `myprogram` with
+* `LD_PRELOAD=/path/to/syslog2stderr.so myprogram`.
+* On macOS, replace `LD_PRELOAD` by `DYLD_PRELOAD`.
+ */
+ /**  
+ *  Copyright (C) 2017-2018, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
 #include <dlfcn.h>
 #include <stdarg.h>
 #include <stdio.h>