ClangTools: Add help context menu entry

...that opens the documentation page for the current diagnostic.

Change-Id: I398fdc82bb118a80536acbb12420a9bac84e66c9
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/clangtools/clangtool.h

@@ -126,6 +126,8 @@ private:
     void updateForCurrentState();
     void updateForInitialState();
 
+    void help();
+
     void filter();
     void clearFilter();
     void filterForCurrentKind();