test: spellcheck :help (vimdoc) files #24109

Enforce consistent terminology (defined in
`gen_help_html.lua:spell_dict`) for common misspellings.

This does not spellcheck English in general (perhaps a future TODO,
though it may be noisy).

test/functional/lua/help_spec.lua

@@ -21,6 +21,7 @@ describe(':help docs', function()
     ok(rv.helpfiles > 100, '>100 :help files', rv.helpfiles)
     eq({}, rv.invalid_links, 'invalid tags in :help docs')
     eq({}, rv.invalid_urls, 'invalid URLs in :help docs')
+    eq({}, rv.invalid_spelling, 'invalid spelling in :help docs')
     -- Check that parse errors did not increase.
     ok(rv.err_count == 0, 'no parse errors', rv.err_count)
   end)