docs: lsp, misc

- Problem: It's not clear for new plugin developers that `:help` uses
  a help-tags file for searching the docs, generated by `:helptags`.
  - Solution: Hint to the |:helptags| docs for regenerating the tags
    file for their freshly written documentation.

Co-authored-by: Yochem van Rosmalen <git@yochem.nl>

test/functional/editor/mode_insert_spec.lua

@@ -84,7 +84,7 @@ describe('insert-mode', function()
       ]])
     end)
 
-    it('inserts named registers literally', function()
+    it('inserts named/clipboard registers literally', function()
       local screen = Screen.new(50, 6)
       -- regular text without special charecter command
       command('let @a = "test"')