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)

runtime/doc/vimeval.txt

@@ -1,10 +1,10 @@
-*eval.txt*	Nvim
+*vimeval.txt*	Nvim
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
 
 
-Expression evaluation		*vimscript* *expression* *expr* *E15* *eval*
+Expression evaluation	*vimscript* *expression* *expr* *E15* *eval* *eval.txt*
 
 Using expressions is introduced in chapter 41 of the user manual |usr_41.txt|.
 

runtime/doc/vimfn.txt

@@ -1,10 +1,10 @@
-*builtin.txt*	Nvim
+*vimfn.txt*	Nvim
 
 
 		  NVIM REFERENCE MANUAL
 
 
-Builtin functions		*vimscript-functions* *builtin-functions*
+Vimscript functions	*vimscript-functions* *builtin-functions* *builtin.txt*
 
 For functions grouped by what they are used for see |function-list|.