vim-patch:8.2.3917: the eval.txt help file is way too big

Problem:    The eval.txt help file is way too big.
Solution:   Move the builtin function details to a separate file.
1cae5a0a03

Note: Neovim-specific references to |functions| were changed to
|builtin-functions|. This included updates to:
  1. test/functional/vimscript/functions_spec.lua
  2. test/functional/vimscript/eval_spec.lua
  3. runtime/doc/lua.txt

runtime/doc/help.txt

@@ -129,6 +129,7 @@ Advanced editing ~
 |diff.txt|	working with two to eight versions of the same file
 |autocmd.txt|	automatically executing commands on an event
 |eval.txt|	expression evaluation, conditional commands
+|builtin.txt|	builtin functions
 |fold.txt|	hide (fold) ranges of lines
 |lua.txt|	Lua API
 |api.txt|	Nvim API via RPC, Lua and VimL

runtime/doc/lua.txt

@@ -17,9 +17,9 @@ an idea of what lurks beneath: >
     :lua print(vim.inspect(package.loaded))
 
 Nvim includes a "standard library" |lua-stdlib| for Lua.  It complements the
-"editor stdlib" (|functions| and Ex commands) and the |API|, all of which can
-be used from Lua code. A good overview of using Lua in neovim is given by
-https://github.com/nanotee/nvim-lua-guide.
+"editor stdlib" (|builtin-functions| and Ex commands) and the |API|, all of
+which can be used from Lua code. A good overview of using Lua in neovim is
+given by https://github.com/nanotee/nvim-lua-guide.
 
 Module conflicts are resolved by "last wins".  For example if both of these
 are on 'runtimepath':

test/functional/vimscript/eval_spec.lua

@@ -5,7 +5,7 @@
 --    null_spec.lua
 --    operators_spec.lua
 --
--- Tests for the Vimscript |functions| library should live in:
+-- Tests for the Vimscript |builtin-functions| library should live in:
 --    test/functional/vimscript/<funcname>_spec.lua
 --    test/functional/vimscript/functions_spec.lua
 

test/functional/vimscript/functions_spec.lua

@@ -1,4 +1,4 @@
--- Tests for misc Vimscript |functions|.
+-- Tests for misc Vimscript |builtin-functions|.
 --
 -- If a function is non-trivial, consider moving its spec to:
 --    test/functional/vimscript/<funcname>_spec.lua