Doc: Changed several files to unificate them... (#6964)

...with those from release 1.8 branch.

os/debian/changelog

@@ -1,3 +1,15 @@
+openttd (1.8.0-0) unstable; urgency=low
+
+  * New upstream release 1.8.0
+
+ -- OpenTTD <info@openttd.org>  Sun, 01 Apr 2018 14:00:00 +0200
+
+openttd (1.8.0~RC1-0) unstable; urgency=low
+
+  * New upstream release 1.8.0-RC1
+
+ -- OpenTTD <info@openttd.org>  Wed, 21 Mar 2018 21:00:00 +0100
+
 openttd (1.7.2-0) unstable; urgency=low
 
   * New upstream release 1.7.2
@@ -1002,4 +1014,3 @@ openttd (0.3.5-1) unstable; urgency=low
   * Initial Release.
 
  -- Matthijs Kooijman <m.kooijman@student.utwente.nl>  Fri, 24 Dec 2004 02:58:47 +0100
-

src/script/api/ai_changelog.hpp

@@ -23,7 +23,7 @@
  *
  * \b 1.8.0
  *
- * 1.8.0 is not yet released. The following changes are not set in stone yet.
+ * No changes
  *
  * \b 1.7.0 - 1.7.2
  *