Ninja: Drop 'experimental' label in documentation

This generator is no longer experimental and has been fairly
mature for several releases already.

Help/generator/Ninja.rst

@@ -1,7 +1,7 @@
 Ninja
 -----
 
-Generates build.ninja files (experimental).
+Generates build.ninja files.
 
 A build.ninja file is generated into the build tree.  Recent versions
 of the ninja program can build the project through the "all" target.

Source/cmGlobalNinjaGenerator.cxx

@@ -507,7 +507,7 @@ void cmGlobalNinjaGenerator
 ::GetDocumentation(cmDocumentationEntry& entry)
 {
   entry.Name = cmGlobalNinjaGenerator::GetActualName();
-  entry.Brief = "Generates build.ninja files (experimental).";
+  entry.Brief = "Generates build.ninja files.";
 }
 
 // Implemented in all cmGlobaleGenerator sub-classes.