docs: multi buffer completion snippet (#978)

* chore(docs) Update recipes.md with multi buffer completion snippet

* docs: improve buffer completion from all open buffers

---------

Co-authored-by: Liam Dyer <[email protected]>

docs/recipes.md

@@ -37,6 +37,29 @@ completion = {
 }
 ```
 
+### Buffer completion from all open buffers
+
+The default behavior is to only show completions from **visible** "normal" buffers (i.e. it woudldn't include neo-tree). This will instead show completions from all buffers, even if they're not visible on screen. Note that the performance impact of this has not been tested. 
+
+```lua
+sources = {
+  providers = {
+    buffer = {
+      opts = {
+        -- get all buffers, even ones like neo-tree
+        get_bufnrs = vim.api.nvim_list_bufs
+        -- or (recommended) filter to only "normal" buffers
+        get_bufnrs = function()
+          return vim.tbl_filter(function(bufnr)
+            return vim.bo[bufnr].buftype == ''
+          end, vim.api.nvim_list_bufs())
+        end
+      }
+    }
+  }
+}
+```
+
 ### Don't show completion menu automatically in cmdline mode
 
 ```lua