Update doc/release-notes/1.6

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Update doc/user-guide/1.6

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Update 'Documentation' link for 1.6

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>