[Documentation] Markdown improvements f to m (#13866)

Signed-off-by: Jerome Luckenbach <github@luckenba.ch>

bundles/org.openhab.binding.mqtt/xtend_examples.md

@@ -1,12 +1,14 @@
+# Xtend Examples
+
 ## Secure connection
 
 In a first example a very secure connection to a broker is defined. It pins the returned certificate and public key.
 If someone tries a man in the middle attack later on, this broker connection will recognize it and refuse a connection.
-Be aware that if your brokers certificate changes, you need to remove the connection entry and add it again. 
+Be aware that if your brokers certificate changes, you need to remove the connection entry and add it again.
 
 `mqttConnections.things`:
 
-```xtend
+```java
 mqtt:broker:mySecureBroker [ host="192.168.0.41", secure=true, certificatepin=true, publickeypin=true ]
 ```
 
@@ -16,7 +18,7 @@ The second connection is a plain, unsecured one. Unsecure connections are defaul
 
 `mqttConnections.things`:
 
-```xtend
+```java
 mqtt:broker:myUnsecureBroker [ host="192.168.0.42", secure=false ]
 ```
 
@@ -28,20 +30,19 @@ The credentials are plain values on the wire, therefore you should only use this
 
 `mqttConnections.things`:
 
-```xtend
+```java
 mqtt:broker:myAuthentificatedBroker [ host="192.168.0.43", secure=false, username="user", password="password" ]
 ```
 
-
 ## Public key pinning
 
 In a fourth connection, the public key pinning is enabled again.
 This time, a public key hash is provided to pin the connection to a specific server.
-It follows the form "hashname:hashvalue". Valid *hashnames* are SHA-1, SHA-224, SHA-256, SHA-384, SHA-512 and all others listed
+It follows the form "hashname:hashvalue". Valid _hashnames_ are SHA-1, SHA-224, SHA-256, SHA-384, SHA-512 and all others listed
 in [Java MessageDigest Algorithms](https://docs.oracle.com/javase/9/docs/specs/security/standard-names.html#messagedigest-algorithms).
 
 `mqttConnections.things`:
 
-```xtend
+```java
 mqtt:broker:pinToPublicKey [ host="192.168.0.44", secure=true, publickeypin=true, publickey="SHA-256:9a6f30e67ae9723579da2575c35daf7da3b370b04ac0bde031f5e1f5e4617eb8" ]
 ```