fix(dir): respect directory buffer lifetime #40453

Problem:
dir.lua leaves previously-navigated directory buffers around.
This is fine by default, but users need a simple way to opt out.

Solution:
1. Respect `set hidden` (via `'bufhidden'`) as one way to make
   previously-navigated dir buffers from showing up.
2. Document a one-liner to hide these buffers

runtime/lua/nvim/dir.lua

@@ -85,7 +85,6 @@ local function render(buf, dir)
     not set_buf_options(buf, {
       { 'modeline', false },
       { 'buftype', 'nowrite' },
-      { 'bufhidden', 'hide' },
       { 'buflisted', true },
       { 'swapfile', false },
       { 'readonly', false },