docs: drop "Can also be used as a method" #24508

Now that we "own" builtin.txt, we cant remove the repetitive mention of
Vimscript's UFCS syntax. It's noisy to mention this for each function,
and it's also not a Vimscript feature that should be encouraged.

Also change the builtin.txt heading to "NVIM REFERENCE MANUAL", which
indicates when a help file is Nvim-owned.

scripts/gen_eval_files.lua

@@ -451,7 +451,7 @@ local CONFIG = {
       '*builtin.txt*	Nvim',
       '',
       '',
-      '\t\t  VIM REFERENCE MANUAL\t  by Bram Moolenaar',
+      '\t\t  NVIM REFERENCE MANUAL',
       '',
       '',
       'Builtin functions\t\t*vimscript-functions* *builtin-functions*',