docs: fix invalid WindowConfig schema, should match to the docs (#1779)

Problem: `cmp.WindowConfig` was defined twice.

Solution: Introduce
  `cmp.CompletionWindowOptions` and `cmp.DocumentationWindowOptions`.
Make fields of these two class consistent with |cmp-config.window.*|

lua/cmp/view/custom_entries_view.lua

@@ -118,6 +118,8 @@ end
 
 custom_entries_view.open = function(self, offset, entries)
   local completion = config.get().window.completion
+  assert(completion, 'config.get() must resolve window.completion with defaults')
+
   self.offset = offset
   self.entries = {}
   self.column_width = { abbr = 0, kind = 0, menu = 0 }