Fixed highlighter resetting of syntax on doc filename change

data/core/doc/highlighter.lua

···
 
 function Highlighter:new(doc)
   self.doc = doc
-  self.lines = {}
-  self.first_invalid_line = 1
-  self.max_wanted_line = 0
+  self:reset()
 
   -- init incremental syntax highlighting
   core.add_thread(function()
···
   end, self)
 end
 
+
+function Highlighter:reset()
+  self.lines = {}
+  self.first_invalid_line = 1
+  self.max_wanted_line = 0
+end
 
 function Highlighter:invalidate(idx)
   self.first_invalid_line = idx

data/core/doc/init.lua

···
   local syn = syntax.get(self.filename or "")
   if self.syntax ~= syn then
     self.syntax = syn
-    self.highlighter:invalidate(1)
+    self.highlighter:reset()
   end
 end