docs: rename builtin.txt, eval.txt #34212

Problem:
Despite the name, `builtin.txt` only includes Vimscript functions, which
is confusing, especially to people who only use Lua to configure Nvim

Solution: From justinmk's suggestion
- Rename `builtin.txt` to `vimfn.txt`
- Rename `eval.txt` to `vimeval.txt`
- The tags `*builtin.txt*` and `*eval.txt*` should be kept for Vim-patches

Closes #33743

(cherry picked from commit 9d5eb3eda5)

src/gen/gen_eval_files.lua

@@ -949,17 +949,17 @@ local CONFIG = {
     render = render_api_keyset_meta,
   },
   {
-    path = 'runtime/doc/builtin.txt',
+    path = 'runtime/doc/vimfn.txt',
     funcs = get_eval_meta,
     render = render_eval_doc,
     header = {
-      '*builtin.txt*	Nvim',
+      '*vimfn.txt*	Nvim',
       '',
       '',
       '\t\t  NVIM REFERENCE MANUAL',
       '',
       '',
-      'Builtin functions\t\t*vimscript-functions* *builtin-functions*',
+      'Vimscript functions\t*vimscript-functions* *builtin-functions* *builtin.txt*',
       '',
       'For functions grouped by what they are used for see |function-list|.',
       '',