fix(lua): pesc, tbl_islist result types #20751

Problem:
- pesc() returns multiple results, it should return a single result.
- tbl_islist() returns non-boolean in some branches.
- Docstring: @generic must be declared first

Solution:
Constrain docstring annotations.
Fix return types.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/lua/vim_spec.lua

@@ -430,6 +430,8 @@ describe('lua stdlib', function()
   it('vim.pesc', function()
     eq('foo%-bar', exec_lua([[return vim.pesc('foo-bar')]]))
     eq('foo%%%-bar', exec_lua([[return vim.pesc(vim.pesc('foo-bar'))]]))
+    -- pesc() returns one result. #20751
+    eq({'x'}, exec_lua([[return {vim.pesc('x')}]]))
 
     -- Validates args.
     matches('s: expected string, got number',